I rarely play coop with randoms. It’s rare for me to encounter outright selfish/dirty players…usually, it’s players that don’t know how to play cooperatively – I’m guessing that’s because they are either too new to the game or are coming from competitive. Other than that, I don’t know…sometimes it’s a mystery to me why randoms do what they do.
Arena and Adventure are my favourite coop modes and my standards are pretty high. I’ll play any difficulty with players of any skill level as long as their style complements mine…I want completions. You can have the bulk of the kills for all I care as long as you’re not constantly getting in trouble.