Why does extrinsic motivation sometimes reduce intrinsic motivation? Why is it that if a person likes subject X but then when he gets paid for it he doesnt like it anymore. What causes this?
This effect has been termed Overjustification effect and was originally reported by Lepper, Greene &amp; Nisbett (1973) who studied the influence of rewards on intrinsic motivated behavior.

Some theories hypothesize that the reward reduces the feeling of self-determination, which in turn reduces intrinsic motivation, because the reward induces the feeling of being controlled by the person that offered the reward. 

References

Lepper MR, Greene D, Nisbett RE (1973). Undermining children's intrinsic interest with extrinsic reward: A test of the 'overjustification' hypothesis. Journal of Personality and Social Psychology 28 (1): 129–137. doi:10.1037/h0035519.