Why so serious?

Ok so if talking to your teamates, joking around, having a good time isnt getting in the way of gameplay, why wouldnt you want that? I understand a situation where people are talking and its getting in the way of them playing, but with that out of the way isnt this game more fun when people are having a good time?

What is the point of telling people to STFU, raging at them, flaming etc. just because they are having fun playing league? I really just don't understand all these "pro" league players who take this game SOOOOO seriously that anything not directly related to the game is shunned.