Why do tennis players bounce the ball so many times before they serve?

The first bounce is to make sure the tennis ball isn't dead, that it bounces properly. After that, it's merely to momentarily take pause. They're mentally centering themselves and physically preparing for the serve. When they feel ready, they stop dribbling the tennis ball, toss it in the air, and execute their serve. Another reason for dribbling the tennis ball is as strategy, partly to psyche the other player out and partly to try to keep them on their toes longer and to possibly catch them off their toes, unready for the serve.

I think it's just a habit. My sister did this when she played tennis.

It is just a mental issue.

All this bouncing the ball (Djokovic and to some extent, Nadal) is just UNNECESSARY. Yes a few bounces to make sure the ball is sound but the rest - either a nervous twitch, or gamesmanship, to slow down the play. With the Oz Final being between Djokovic and Nadal - make yourself a good snack because the match could take FOREVER! I just hope the umpire keeps a handle on the 'time between'.

I always did it as stress relief before my serve rather than just walking onto the court and into the serve.