I can understand rage quitting when you end up in a "bad" team, where no one communicates, captains don't work together and you get stomped by the enemy team. I don't rage quit, and I think it puts people in a bad mood to be losing and to see everyone leaving, thus encouraging them even more to leave. We shouldn't forget there is a Surrender button ( but if nobody communicates, I don't see how they will all agree to surrender ).
Maybe on of the reasons people ragequit is that quite often, I see tons of beginner matches, but when I play in a regular lobby, there are still some totally new people that join, and if the teams are not balanced to counter it, they end up in the losing team and it causes some ragequit.