Let me preface this with a few things; paladins are one of my favorite character classes to play in D&D. The allure of the class is, to me, to create a distinctive personality and background within a very rigidly defined archetype. That, and being able to whup ass on evil, in particular, better than anyone else. The paladin is a "fighter without bonus feats" against things like elementals, animals, and the like. But when fighting evil...a creature or person deliberately out to harm others...the paladin kicks ass above and beyond all others.
And when I play D&D, I want to play a hero. Usually. Playing evil characters tends to bring out facets of me I'd rather not acknowledge. Of course, any class can be a hero (excepting bards; nobody believes a bard can be a hero), but the thing about the Paladin is, he is destined to be. It isn't a choice, and it may not even be what he wants, but it is what he is on a more fundamental level than any other class.
Now, I've noticed a trend in recent supplements and such to introduce "variant" paladins. I don't necessarily have a problem with variations of paladin abilities, but most of these variations seem to be on the order of "letting people play a paladin that isn't LG." There's the CG paladin, the LE paladin, the CE paladin.
And that's all nonsense. Rot. Swill. Garbage. Stupid. Fluff. A CRUTCH FOR WEAK PLAYERS WHO CAN'T HANDLE THE GAME.
Ahem. Sorry. Was channeling Weird Pete for a second there.
Point is, Paladins are Lawful Good for a reason. For several reasons, in fact.
Let's start by examining the origins and meanings of the word "paladin." Without too much linguistic nonsense, paladin appears to have originated around 1595, referring in French literary romance to the 12 knights in attendance upon Charlemagne. Other less specific definitons; a paragon of chivalry, a determined advocate or defender of a noble cause...most of the rest of the definitions proceed from there.
Now, how does this relate to Paladins being Lawful Good only? Patience. We'll get there.
One of the arguments I often here for the variant alignment Paladins is, "Well, other gods have champions, too." Sure they do. They're called CLERICS. Or, for that matter, rangers or wizards or druids. If you want to become a super-duper CG champion, you work toward the "Holy Liberator" PrC.
Furthermore, folks, Chaotic Goodness is not a cause. Chaotic Good is the alignment that says "don't start none, won't be none," or "stay away, leave me alone, let me go my way." Mal Reynolds of Firefly is the perfect example of CG. He follows his own needs and conscience as the situation dictates, but isn't really out to change the world (not usually). He's a good man, sometimes even a very good one, but he's not a Paladin, and he'd laugh if you suggested he was. Chaotic Good is, by and large, a reactive sort of alignment, and Paladins are by their nature an active class. Their mandate is to actively hunt down evil and eradicate it. Chaotic Good characters aren't generally out to do that. They're individualists. Not crusaders.
By extension, CG gods aren't the kind who'd want their followers out championing causes, are they? I mean, combating evil where you find it, sure...but are they likely to grant significant power, like the paladin wields? I don't know, but I don't think so. Again, it's a matter of active vs. reactive. Chaotic Good beings react to threats or perceived injustices or crowding of individual freedoms; are those the kinds of things a Paladin does? Well, yes, but not as a cause. I just don't see it as possible to define "Chaotic Goodness" or CG deities as "a cause" or
As for Lawful Evil paladins? Still not buying it. Lawful Evil is not so much a cause as it is an alignment geared towards acquiring personal wealth and power. But again, anybody can do that. You don't need special abilities to make you a dominating conqueror, not really. And there's the Blackguard PrC to cover the situation of the "anointed of a dark god." And I have no problem with the Blackguard PrC filling that role, and it makes sense to me, for several reasons. For starters, dark gods aren't, I don't think, likely to grant those kinds of powers to someone who hasn't gone out and proven themselves devoted to the cause; I don't see 1st level characters in the "blackguard" role. Secondly, there are specific rules about the PrC that pertain to Fallen Paladins who take up the role, and that makes all kinds of sense to me. Nothing a dark god likes better than stealing the champion of his enemy.
I don't see a CE god granting those sorts of powers lightly, either, which is why I also don't buy the CE paladin variant. Look, imagine a dark god, d&d style. They are an eternal and inexorable and ineffable expression of all that is evil in the nature of sentient beings. They aren't going to share their power lightly, not without tremendous consequences to the sharee.
Contrast this to your typical LG god. They're exactly the opposite, and, I feel, much more likely to grant powers to someone based on their potential. Someone has to go and combat the evil out there, right? And while the representatives of LG gods can spend the time and energy to train and shepherd and educate those prospective, the deity in question needs to equip them with the ability to do what they are Called to do. I just think it's hard to make an argument that someone is Called to do evil in the way a Paladin is Called to do good. Most evil arises out of self-interest, of a desire for more personal wealth/power/magic, whatever, and rarely does it rise (or sink) to the level of proselytizing and actively seeking to spread the word and 'erdicate good.' Sure, there might be some few truly, truly devoted who act in that manner, but...are they going to go seek out and train their replacements, people they'd have to share power with?
A keen example here is the Sith (chorus of boos from all of you prequel haters; just trust me). There are 2 at a time, because otherwise, they get overwhelmed with backstabbing, plotting, assassinations and the like, and nothing gets done. The way a new Sith advances is by killing one up top. Don't share power. Don't make it easy for your replacements. Is this the kind of environment that fosters lots of individuals with potential to become great champions of evil, the way most Campaign Settings seem to picture Paladinhood?
No. But it is a way to foster the role of something like the Blackguard, a champion of evil who has proved himself by...being a champion of evil. I have no problems with the concept of an "anti-paladin" being applied to a PrC. But not a base class.
There's a game balance issue here, too. Paladins (along with Monks) get the most class-centric abilities, things no one else can effectively do. Most of these, for the Paladin, come at relatively early levels. By level 6, a paladin has gotten all of the abilities he's going to get; Aura of Good, Smite Evil 2/day, Divine Grace, Lay on Hands, Turn Undead, Special Mount, and Remove Disease 1/day. If you don't want the remove disease too badly, you can just go to 5. If the paladin doesn't have severe alignment restrictions, then min-maxxers might just take 5 levels and never go back. Some of them will do that anyway. But if they're a Chaotic Good Paladin, they can go and take levels in, say, Bard or Barbarian. The former, being so Charisma based, stacks especially well in a numbers way, but doesn't make any damn sense in a roleplay manner.
Finally, there's the challenge of playing a Paladin. Making a Paladin CG removes the very challenge that makes playing a unique Paladin so compelling. Look, I've played many a CG character in my life, but let's be honest; it's the easiest good alignment to play well, and might be the easiest in the game to effectively manage. You don't have carte blanche, but it's close.
The thing is, most people picture the Paladin as "Lawful Stupid," inevitably losing their life in a foolish cause, or being a prig who has no sense of humor and lives on bread and water. The fun of playing a Paladin is challenging that assertion. Lawful Good doesn't offer nearly as much latitude as anything else does, but that doesn't mean you can't have personality or that you have to be a stiff-necked prig. For example, one Paladin I played for a long time spent a fair amount of free time at a brothel. Not as a customer, mind you, nor in a paid position, but he hung around because, fairly often, the women there needed protection, or a little 'cure disease,' or someone to help them out in a number of ways. Maybe what they were doing wasn't legal, but the goddess he followed stressed caring for the younger, the weaker, those without means to protect themselves, etc. He saw that he was doing that, didn't accept money for it, didn't sleep with any of the girls (though Paladins don't have to be celibate, far from it) and all in all, helped make it so that the lives the women in that brothel had were better than they otherwise would've been. Maybe he preached a little bit...if he saw an opening...but not too much.
Anyway, enough character reminiscence. The point; Paladins are Lawful Good for a reason. For several reasons. They shouldn't be anything else.