Why do we choose what we choose in competitive play?

Ultimately, what made me pick Shulk over Ike in Smash at the end was merely, that he is difficult to play and because I found his Monado arts pretty interesting. Originally, I went with Ike, just because I saw some Youtuber playing with him and even now I like randomly picking Ike, though, I'd still pick Shulk most of the time, because of his interesting playstyle.
Back when I played Dissidia I used to play a bunch of characters...