Xelloss and Filia: Reasons 
Opposites Attract!
Good/evil, open/secretive, temperamental yet kind/cheerful but selfish... As opposite as any two people can be. In Slayers, all of the couples are based on contrast. Basically, the Slayers couples are more compatible because of their differences. The fiery and intelligent sorceress Lina who can easily get out of control is tempered by the easy-going, if not exactly bright, swordsman Gourry (though I believe he is smarter than he appears.). The shy, depressed, chimera Zelgadis whose motives tend to be a bit selfish needs the ever cheerful princess Amelia to bring him out of his shell. Needs her to give him acceptance for who he is, and force him to think about others and see the good in the world rather than sulking in self pity. In the same way, Xelloss and Filia are perfect compliments for each other. They will probably never agree completely since they come from such opposite perspectives, but their differences can serve to make them stronger. (Ophelia's wrote another good essay on this topic Xelloss Abode: Essay.)
They Fight Like A Married Couple.
If they are going to fight like a married couple then don't you think they ought to get married? Just because they are fighting doesn't mean they hate each other. In fact, there are many couples out there who express their affections by arguing. This may not be the most ideal sort of relationship, in this case, it is downright terrifying the damage that they can do, but there are very few ideal relationships in the world.
Mace-Sama + Masochist Demon!
What more could you ask for!?! A masochist and a large, blunt weapon. Xelloss, as a mazoku, enjoys negative emotions, and Filia is an easy target to provoke her anger. He certainly appreciates her destructive potential, going so far as to compliment her after she destroys a town. But even that only makes her angrier and she tries to kill him. Which leads to another point: Who else could even survive a long term relationship with Filia? Gourry can handle Lina's abuse, but Filia is much stronger and easily loses control of her temper. Zelgadis might survive with his stone body, but he certainly didn't appreciate the beating he got when Filia made him do a ping-pong imitation. But Xelloss can easily dodge her attacks, and even if he does get hit, it won't do much damage. He might even enjoy it...
Xelloss Is Always On Filia's Mind.
Episode 5 is a perfect example of this. The entire episode Filia simply can't stop thinking about him. When he shows up during her fight with Lina, Filia immediately forgets everyone else to focus on blaming Xelloss. After she fled she was so focused on his words she allowed herself to be captured. In prison, Filia continued to rage at Xelloss instead of looking for ways to escape. This continued until the end of the episode when his taunting was what set her free. Ahhh, so romantic... (¬_¬ whatever...)
Filia Is The Only One Who Can Make Xelloss Mad.
In addition to Xelloss making Filia mad, she is the ONLY person who can make him angry in return. There were some interesting essays at RoyalE ColourS that discuss how the higher level mazoku do indeed feel emotions but over time it becomes hard to find anything to stimulates them. Well Filia certainly can get Xelloss worked up! Even to the point of yelling and twitching in annoyance!
They Do Have A Few Tender Moments.
First is the scene when Xelloss offers to help against Almace at the Fire Dragon Temple. He's been watching Filia, as usual, and when she makes a stand against her Elder, he appears at her side and claim to support her. This is in part an effort to embarrass her, but that doesn't explain why he remained hidden right until Filia joined in the rebellion.
Next is the ever-cited scene when he saves her from the falling rocks only to drop her on Valgarv. People argue that he couldn't let one of Lina's friends die, but why not? Especially since he has just offered to kill Lina herself. After that Xelloss is badly wounded and Filia comes to his aid. Lina may have ordered her to help Xelloss, but Filia is stubborn enough she is not easily bossed around. And no-one told her to hold Xelloss so close to her chest (...heart? hehe. ). They are awfully close for quite a long time during that first climatic battle, and neither of them seems to mind it at all.
Much later Xelloss takes Filia hostage and threatens to kill her if Saichuro doesn't co-operate. Xelloss is such a skilled manipulator he should have known what kind of threat would work. But he was wrong and Saichuro refused. Why would Xelloss so grossly overestimate Filia's importance to others unless she is that important to himself? At this point Xelloss lets her go, instead of keeping his threat. Xelloss is not the type to bluff. Unless he has some other motive for wanting her alive...
Finally I cite the scene where they are fighting Darkstar and Xelloss and Filia must combine their powers. When Filia finally agrees to it Xelloss gently teases her (In the dub version he even says "That's the dragon I love.") and Filia smiles back at him. That is quite a little smile to give someone you hate!
They Both Like Tea!
It is good for a couple to have some common interests! Filia is downright obsessed with tea. But Xelloss has a thing for it too. During Next he regularly eats and drinks in order to keep the appearance of being human. Even so, he doesn't stop drinking tea when his true self was revealed. I believe tea is the only thing he eats or drinks during Try. I can just see these two having a peaceful tea party together...
Xelloss: Ahhh, Filia, you certainly can make nice tea...
Filia: Really? *blushes* Thank you!
Xelloss: ...for a dragon.
Filia: What Was That!!!!!!!! *reaches for mace*
Xelloss: I was just thinking how odd it is that such a violent dragon can pretend to be so innocent and peaceful when drinking tea. We really do have a lot in common... *hit over the head with Mace-Sama*
They Aren't Really As Different As They Seem.
They have a lot of things in common aside from their favorite beverage. They seem to even think in the same way. Just look at the marriage ball episode where they both reply with exactly the same line when they are paired together! And they do seem Awfully upset by this. They deny the pairing with a vehemence to be expected of someone who refuses to acknowledge the truth.
They Can Begin To Bring The Mazoku And Ryuzoku Together.
Ok. This will probably never happen. But you never know. In Try there are quite a few references to combining mazoku and ryuzoku power. In fact the whole plot with Valgarv and Darkstar questions the reasons for the conflict between light and darkness. It asks the question of whether a new world could be founded where such conflict wouldn't be necessary. Of course the characters don't like the idea of destroying the world to attain that goal. But the question remains. Is the fighting really necessary? Is there some better way to attain that new peace?
They Look Good Together
This isn't as silly as it sounds! When animators design characters they generally keep things like this in mind. These two definitely go well together! This is reaffirmed by all the pencilboards, commercial break scenes, and other promotional materials showing them together. Their dominant colors are complimentary because they are exact opposites on the color wheel: Purple/Yellow, Black/White. This is also a good reflection of the characters themselves and how they are opposite, but complimentary.
They're So Cute When They Fight!
You know how that goes when the little boy who picks on a girl in school really has a crush on her. Well Xelloss and Filia certainly like teasing each other. And they are so darn FUNNY when they fight!
They're About The Right Age.
Not only are they both far older than the other characters, but their expected life-spans are much longer as well. Since they both live practically forever, they will have plenty of time to get used to each other ...and even fall in love ^-^
