This is a discussion on Taking over the betting within the online poker forums, in the Learning Poker section; It's very typical for everyone to wait to bet on the flop for the original pre flop aggressor to make his continuation bet
If someone 'steals
I think, it can be all of this. Player could have hit the flop, that he wants to protect. Maybe he wants you not to bet and control the pot. Maybe he is drawing or something. Or a player wants to know, if you had hit the flop. If you was stealing, he can have a read on you and wants to steal back. I think it depends on the situation. How is the flop formed, what kind of player type is against you, what position you both have. Maybe also one is just not knowing, what he is doing. It can have many reasons...
I think sometimes if you let someone steal, you are showing them enough weakness that you can put the hammer to them later. Betting should be played like chess not checkers. Set someone up for what you want to accomplish later.