God I hate losing when online. I can stand it if I am just playing a single player game, but when someone is kicking the shit out of you online, it just gets annoying.
Especially when it's not really how skilled they are, it's just the circumstances of each run. Like when I play sniper, and I'll have Scouts and Soldiers jumping onto my platform. I'll switch out of sniper mode to deal with them, and get the enemy sniper across the way gets a head shot. SO fucking annoying when it happens in a row.
Usually what I do though is sit and wait a while doing nothing before I disconnect, so that the person doesn't think I left the game because they were whipping me.