you have to leave because of irl stuff or whatever something important.
a bug/glitch is happening that wont let you experience the game in a good way, for example, a hud bug, a bug that wont let you use weapons effectively, etc.
deserting all maps your pc cant handle, because well, is too much, even at shit settings, if my pc is acting ass because of a certain map, im leaving, im not gonna torture myself in 5 fps hell, yes ive tried lowering settings, no they dont do jack shit.
deserting a battle because of a map/mode you dont want to play on, for example, the disgusting dday, train escort game mode , etc.
also, do not say im evil or bad or whatever the hell, is a game for fucks sake, not a sport, so what if my team has 1 less human, is a game ffs, not a SPORT, also, games are meant for fun, so im gonna play for fun, im NOT gonna torture myslef, and time is VERY important to me, since i value it a LOT, so ofc im not gonna waste my time and torutre myself on some ass map, I WILL NOT PLAY DDAY (unless is lowkey fun fr)
any more things that are also valid to deserting games?