Whats wrong with picking a champ because I like him/her?

Because not everyone picks a champ just because they like them. There's a reason champion popularity spikes massively when they become strong and drops massively become weak - people also pick them for competitive reasons. A champion needs to be fun enough for people to pick them cos they like them, but they also need to be unique enough so that people can pick them because they think they'll do well in a game.

There's nothing wrong with picking a champion because you like them, but you should have a reason to pick them beyond that.

The lcs my friend. Because whatever the pro's say, its true -.-

Because it makes the game stale, as well as potentially unfun for others. The goal for this class pass overs is to retain the style of the champion, while differentiating them enough that the non-champion x main has a reason to pick champion x over champion y

So the other 4 people on your team are interested to see why you picked that champion and how they can use your champ's strengths to their advantage instead of thinking "oh he's just playing that bad version of Brand because he has a hard-on for beholders"