Lee sin is one of the most difficult champions of the game, so if you want to be main Lee Sin you need a lot of effort. You need to be constant, i mean that you need to play lee, lee, lee... I know that is very difficult, because anyone get bored when played a champion 5 or 7 times. One the tips that i give is that you need to be patient, the first 10 games you are going to get bored and say "I suck with Lee, I never going to be good with him".
I've been playing Lee since the beginning of Season 5 ( 1 year), and I considered a normal player. So as I said is very difficult, but if you commit, you can achieve be a Main Lee Sin, the secret of that is only practice.

You pick Lee Sin in this cases:
One case it could be if you have an specific comp, like a Yasuo for example.

The another reason is because you are so good playing Lee sin, and you think you can snow ball the game in the early game, because the potential of Lee is in the early, so if you carry the game since that time, you won the game 100%.