Why do people even leave ranked games

I don't mean disconnects I mean why do people actually leave the game. Like, you can't go play another one till this is over, what are they gaining? Two people saw fit to leave my games today, one over the choice of support, the other for unexplained reasons, and I lost all the progress from my two wins that I had earlier. Each time, we were actually ahead towers, kills, etc. and they left, and I really just want to know why more than anything