There's a long discussion about it in Unconditional Parenting
Or here is an article by Alfie Kohn.
But basically the argument goes that you want a child to be 'intrinsically motivate', ie doing something for the pleasure of doing it, rather than 'extrinsically motivated', ie doing something for a reward.
And even if a child starts off intrinsically motivated, by rewarding them you can turn them into being extrinsically motivated.
Being extrinsically motivated is no fun. You don't get pleasure from the task, only from the reward. The task turns into a chore....
For example say a child enjoys drawing, and draws you a lovely picture. And then you say 'well done'. Next time they might do a picture in order to hear 'well done', instead of because they like drawing...
However if instead of saying 'well done', you say 'whats the picture about? Why did you choose purple for the tree?....' and have a discussion about it, then hopefully they can keep up their interest in drawing because they like it....