Like others said, chemistry, though I'll add to that the "they complete each other" thing matters to me too, though I don't think I'm looking for "completion" per say. I don't think any other person can complete you, but I like to see partners bring out something positive from the other person. Like, I can get on board with a certain amount of bickering/enemies to lovers, but if there isn't something positive that comes out of the pair being together, then I'm out. Not a fan of unhealthy relationships.
Along with that, I LOVE a well-done enemies to lovers trope, but I HATE a badly done one. By that, I particularly mean situations where there are shades of romanticizing abusive relationships, I'm out. I don't want to see them punch each other or really cut each other up verbally if I'm supposed to root for them to get together. I don't like it when a girl slapping a guy is played off as "cute" one little bit. If they are going to physically beat up on each other at all, there better be an incredibly good reason for why they are able to patch things up eventually. (some forms of soldiers on opposite sides of a war can work, for instance, because the violence is never personal) Essentially, if there is a personal/sexual aspect to any violence in a ship, I NOPE out real fast.
In terms of chemistry, nothing makes me root harder for a pair faster than it being two dorks who love and support each other but just can't admit their feelings for REASONS!!!! Whatever the reasons. There can be so many good reasons. That kind of emotional torture is my game.