For me it's almost always about who's closest. I'm playing a game on noble right now and I'm going for conquest. Mansa and Cyrus were closest, but Toku and Izzy were the most powerful. I went for Mansu and Cyrus first, and now Izzy's on my north border so I'm going after her.
If I weren't so much more powerful than Toku (second strongest in the game) and Ghengis, I would've sent some pillagers their way to take out infrastructure and chip away at their forces. But there's no need for that as I'm way more powerful than both and the gap is only getting larger.
In a more peaceful game (space race or especially cultural), war is only fought defensively, pillage-style to set a rival back, or as a land grab against someone I know I can beat and who I know others don't like very much.