A Battle Of Wills
I wasn’t really into slash until one day I realized that Tony and Loki make perfect sense. The full-tilt divas couldn’t be happier than when they’re up against a worthy opponent. I watch Tony teasing Justin Hammer and just imagine him thinking “This is too damn easy. He falls for every little thing.” He had to build Jarvis to have someone quick enough to spar with him verbally.
As for Loki, he wasn’t even interested in fighting Thor one-on-one until his brother’d gotten some sense beaten into him and Thor came back with that hammer, officially “worthy.” But after that, he goads Thor into fighting him with a vengeance. He’s been searching the universe for someone who is his match, such that neither can make the other kneel.
They balance each other quite perfectly. Vast brains and not a slouch in combat, but all their best physical weapons are based in intelligence too.
They’re both great appreciators of the game that on Asgard would be called flyting, and on Earth verbal sparring, or simply snark. There is nothing better in setting up tension in a FrostIron fic than a nice bout of mutual witty insults. They might not talk to each other for long in the Avengers, but they’re smart enough that they learn a ton about each other during that exchange.
They’re both masters at hiding the pain that their lives have inflicted on them, the inadequacy that they feel every day. They’ve never met anyone who would really be able to understand all that they carry with them.
There’s often a tension there, in fics, as each tries to pull out the other’s story first, because it’s hard for them to trust each other until they can see how similar they are if you dig down to that core of pain and self-doubt.
It’s often said that Tony could see a lot of this in Loki when they had their chat in the movie, that he recognized the scramble for power as a) a cry for attention and b) a means to another end, rather than an actual desire to rule. He recognized that, being a man at the head of an empire for the simple reasons that he wanted to make his father proud, and he wanted enough money that he could play with his toys all day. The company itself interests him very little.
Genius Level ‘That Kid’itis
The other thing that they’re always looking for is simply someone who is anywhere near as intelligent as they are. It’s hard to develop a meaningful relationship with someone who can’t follow your thoughts on, say, interdimensional energy phenomena or what have you. I think it’s safe to say that growing up, neither of them really had best friends the way other kids did.
It’s often fun to follow up on the trail of hints provided that connect the time Howard Stark spent with the Tesseract with his ability to conceive of a supernaturally efficient power source of his own. This allows us a beginning point for common ground between Tony’s science and Loki’s magic (already established with a comment by Thor as being related).
After that it’s only a short jump to “Finally somebody gets it!”
Push and Pull
What it comes down to is that they interact in such similar ways with the world around them, when they meet, something dramatic is bound to happen. Either they’re pushing each other off the edge of their mutually questionable sanity, or they are pulling each other back from the brink. There are so many viable scenarios.
And it’s oh so entertaining to watch it happen. They are the cleverest, most interesting characters in their universe.