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 & 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.
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.
To understand Extrinsic motivation to do something think of the carrot and the stick theory. So one only does something because they have to, ie, for some reward for due to some fear of penalty. Think of when our parents "made" us do the dishes. Thats an example of an extrinsically motivated act. Whereas Intrinsic motivation is when we do something because we want to. So sometimes what starts out as an extrinsic motivation becomes internalised within our belief system and it actually becomes somewhat intrinsic. for example, our parents instil in us to brush our teeth every night and then when we become parents that becomes our own own intrinsic value. however without going into too much detail, it is still an extrinsic motivation while being classified as a subset of extrinsic motivation because it has almost the most of the elements of intrinsic motivation the only difference being is that it is an internalised extrinsic motivation. So to address the question, the extrinsic motivation has not interfered with the intrinsic motivation. A study showed that once a reward was offered to people who were intrinsically motivated, they actually lost interest. however when praise was offered instead of reward, the intrinsic motivation was maintained or increased.
References Deci, E., & Ryan, R. (Eds.), (2002). Handbook of self-determination research. Rochester, NY: University of Rochester Press