How do you pick your main(s)?

So I'm the opposite of a OTP, I'm pretty mediocre on everything lol. I have 1400 battles but not a single mon over 100 battles. Whereas I constantly see other people in masters with hundreds of games on one or a few mons.

So I'm curious, how did you pick your favorites / what about them made you want to stick with them? How did you improve from playing so much on the same mon?