My main reason is computer issues [I don't play console]. A computer issue that is being worked on and I think it is fixed, only for it to cause a crash right in the middle of something that takes ages to accomplish in the game.
There is also server-side computer issues in mmorpgs or problems like lag and server crashes that make me insanely angry so I turn off the game.
Trying to get through an extremely difficult single player game's section and after having learned every possible thing to learn, I still cannot get it right and resort to reading cheats and still fail I will rage quit.
I think a person's behavior in a game can sometimes relate to their real life personality, I say this because mine does especially in mmorpgs and less in games that are single player. In single player games, I am less concerned about how my behavior effects others [because there are no others] so I am more likely to endure [or stick around to resolve] frustrating situations.
As for interacting with other players, I am usually the mediator type so it is rare when people or working with other players will get me to rage quit, especially if the players in question are lacking in a skill that I have. I will probably be the one patiently explaining to them how to do it right.
So interacting with people only causes me to rage quit if it is a very caustic very extreme situation that even I cannot resolve. This relates to abusive players and abusive environments in game that I cannot escape without quitting the game for the moment. Also RMT stealing something or directly ruining my game play can drive me insane to the point of rage quitting. Generally things that are completely out of my control or ability to resolve or improve upon.
Rage quit in general could also relate to the source of the frustration from the perspective of the game player. They are angry while playing the game so they stop playing it to get away from that source of anger.
I am curious how many female gamers rage quit compared to male. I am female btw, rage quitting is rare for me.
Edit: I agree with Kuishen on the spawn camping but add that if they are other legit players [not working an RMT shift] it will not piss me off any where near remotely close to RMT spawn camping. I do have a sense of letting others have their turn that allows for really good patience. Being passive and not a 'get leet gearz first' type, I will wait for days until something is not popular to try it and usually this removes the regular players spawn camping where the RMT will still be doing it and manage to get me to rage quit.