Imagine waking up on a Saturday morning, the suns out and you think to yourself "Wow, it's a nice day, I think I'll go outside."
But you can't. You're in a league team, and today is going to consist of hour after hour of TF2, playing the same old map with the same old players, getting slowly more bored, and slowly turning a game you bought into a chore, or a job which you paid for.
This keeps happening, and then you start competing properly. At this point you no longer even enjoy the game, you're just competing because so much training has drilled it into your head you're too good for pubbing, and you will only get a challenge from other teams. Something you used to enjoy is now more boring and repetitive than your school, work, or any other commitment.
That's my opinion on leagues and professional gaming in general.