Why do we become bored of stimuli after multiple exposures to it? Why do we become bored of songs, films, stories, jokes, food after being exposed to them or experiencing them multiple times?

Why is it when our cortex becomes adequate at predicting the outcomes of a song, film, joke it seems to be less enjoyable, is this the case? And if so what is the neuroscience behind this phenomena that stops our pleasure center being activated? I get the evolutionary aspect of why this maybe the case but I wanted more detail on the neuroscience side.
This will probably only answer part of your query.

I think that this is an example of 'habituation', which is a decrease in response to stimuli.  There has been some research in this field, a particularly good reference is "Habituation: the 
 difference between
 familiar and unfamiliar
 stimuli", where the authors contend that the underlying cause goes beyond 'sensory adaptation' and 'muscle fatigue', and that the reasons and neurological responses are quite unique for each situation.

An important point from the reference above is:


  There is in fact a wealth of examples from a wide range of human and animal behaviour where isolated responses wane as the stimulus which provokes them recurs, without the intervention of muscular fatigue or sensory adaption (Hinde, 1970). However, the scope of habituation is so comprehensive, embracing almost every type of behaviour, that as Hinde says the `underlying processes' must vary enormously. 


I hope this helps.