Mick never seems satisfied with his achievements. Don't get me wrong, he is not depressed or anything. He does celebrate those achievements, but his achievements become normal after some time, and he moves on to the next big thing. This is best explained in terms of the:

Adaptation level phenomenon.

Explanation:

Adaptation level phenomenon is demonstrated as the propensity to swiftly adapt in a new situation before the situation turns into normal. It is associated to human tendency to quickly make judgments (of lights, of income, of sound) in relation to the neutrality defined by a previous experience.

The given description exemplifies 'adaptation level phenomenon' as his tendency to 'celebrate his achievements' reflect his 'adaptation of a new situation' before its gets normal and another new experience is required to evoke our senses and excitement (as reflected by his 'moving on to the next big thing'). Therefore, 'adaptation level phenomenon' is the answer.