For myself, the answer is multi-faceted:
1) Many a time questing while fun with friends you know can be more efficient solo. sometimes I feel when parties have to wait up for me, or I have to wait for them to catch up, maybe re-equip, or get distracted by side items like mining an ore or something.
2) Sometimes dungeons or quests can almost become too involved where you need dedicated people upfront. In WoW, sometimes if a party gets wiped once or twice, members start dropping from the group and the search for more to fill in becomes a time sink and/or a challenge for other peopel patience.
3) Rude players - Often I get spammed over and over to help someone with no please/thank you's, etc. Once the quest is over, or a certain section they need is 'over' or completed...they're gone often leaving you alone anyway.
4) Team member requirements....Often raids all become about DPS or "We need 2 tanks, 1 mage, and 1 healer"...not "Anyone wanna play for fun?" If you don't fit the requirements, you aren't even considered.
5) Too much trash talk. When someone in my team complains and complains and complains about others..like "Stupid Noob", I don't want to play with them. I'd rather play with a Noob then someone who berates them every five seconds.
There's more, but there's a strating pint to chew on.