The game is extremely addictive. When you start getting serious at this game, it means that you will be offended by many small things you wouldn't care about if you were just a casual gamer.
Let's say you play at a high level game. You do well at your top lane with your carry, you farmed well for 10 minutes, even got a kill and you're satisfied with how the game's going so far.
Meanwhile you have your mid lane getting crushed by the enemy. Your ally mid dies 5 times in 8 minutes and you soon get ganked by a level 12 middle hero when you're only level 6. You get run over as well as they're little you can do.
The rest of the game snowballs like this and because you can't give up in DotA 2, you have to play for 15-20 minutes defending or not defending your base against feeded (too strong) opponents. So yea, that's when the unfriendliness comes from