When I picked up smash, I really thought I would just default back to Mario or fox, since they were both always the same in almost every game. both very solid, very strong characters to play, I didn't really think I had to worry about deciding who else to play.
And that was fine, for about a day or two, but I don't know there was something I felt like I was missing from just playing these two characters again... so I just decided to play around with the others newer characters, and some old that I felt were more comfortable.
I fell in love with Greninja and shulk, they both just held this sort of solid feel that I liked out of the characters I played, but they also gave me something new to try out and mess around with, and that sort of sense to take a character and just want to get better with them what drove me to sticking with Greninja and shulk.
You don't really choose a character to play, you have to want to play them, and when you find that drive, you'll progress faster because you'll learn the character a lot better if you truly want to get better, so you've got to really want it.