It's the same phenomenon as road rage. You expect certain actions out of people without communicating, and when they don't conform to your expectations, you wonder why they can't just see the most obvious thing (to you).

There are different reasons for each, but very similar feelings.

I don't personally get road rage because driving isn't competitive. But I do react poorly to others raging at me (which is also a childish problem I should get over.)

Game specific diatribe incoming. Stop reading now or forever hold your peace.

Partly it's that I have some unique ideas about how support can be played. Partly it's that I'm absolutely more inclined to support somewhere else if I've lost confidence in my "partner". Partly it's that I'm actually bad at very obvious parts of the game, while it's easy to overlook what I am accomplishing. I'll take too many risks to maintain vision, for example. People get mad when I die stupidly, but don't see how they've had better than professional vision for the entire game. (Because I take too many risks for it, and bad players are even worse at clearing vision.)