Deadlines are the best things ever for me. It's not because I need someone to force me to write; I love writing. I do have a problem with writer's block, but it has to do with my main reason. It's because if I don't have a deadline, I spend too much time trying to make my writing perfect.
I'm not denying that taking time makes a near-perfect story. You can read account after account of authors that took years to write their wildly famous stories. The thing is, though, that you can't be too focused on perfection. Your story will never be flawless; with more and more editing it gets better, but never impeccable. There's a point when you just have to say that you're done working on the story, because you can't work on it forever. You lose experience and opportunities that way.
The more stories you work on, the better they get. You won't always feel that way; you may like the way you built a plot in a past story, or the characters from another. With every new narrative, though, you gain experience that you didn't have previously. Deadlines make you work hard to finish a story in a certain amount of time, so that you can move onto the next adventure and give it your all.