I have a ratio of wanting to rage quit .. bout 10 times in 1 game.
At that point i press quicksave, quit, make some coffee and, eventually, leave my flat.
Never rage quit, but often fail to finish games. Save a game mid-way, come back a few days later and find I've forgotten what I was trying to do, or the game doesn't seem interesting, or someone has posted a more interesting save, or posed an interesting question that requires an in-game test, etc. etc. Then a month or so later, when cleaning up my saves folder, I delete the save because I have no idea what that game was about.
In MP, I never quit. If a player in MP quits before his civilization is eliminated, he lands on my black list.