I think it essentially comes from the nature of the game: as an individual, you're suddenly stuck with a bunch of other people you have to cooperate with. It's not like tf2 or CS. The teams are smaller and the stakes are much higher. People care about winning games in DotA a lot more than they do in most FPS games, where the journey is the destination. In DotA, the win is the destination and it's the validation of your abilities (k/d ratio doesn't cut it).
You would think having the win be so important would make it easier or better to work together, but the fact of the matter is most DotA players suck. Even people who consistently win public games still suck. Players hold everyone else to incredibly high standards and get frustrated when they feel like their team "drags them down." This is compounded by a feeling that, if you're doing badly yourself, you should scapegoat a team member in order to shift the blame.
DotA 2 is actually a lot better. I'm not sure why exactly, and it depends on your level. Maybe there's hope.