why do people get pissed when the first pick a champion

I hate first pick mid's and tops. Easiest lanes to counter.

First three picks SHOULD be sup jung and adc. But selfish reasons

Personally, I don't like being first pick just because I know that the enemy will try and counter me. It's to be expected though, and isn't really anything to complain about.

Basically, the answer is that they are probably the immature players and would find something to complain about anyways.

On the plus side, I think I have more experience playing Nasus against Vladimir, Darius, Renekton, and Heimerdinger than any other matchup at this point. I tend to do relatively well against them by now simply due to how many times I've had to play against them.