My BIG Question

There is a quote that I have always remembered. I forgot who its by, and I don't remember where I first heard it but it is -- "the problem is you think you have time" -- This is relevant to my "big question" because my Big Question is: WHY DOES TIME GO BY SO FAST? ...  I have always wondered why time goes by so fast when you are enjoying yourself (summer break, vacation, hanging out with friends) but goes by so slow when you are not enjoying yourself. Does your mind totally forget about the aspect of time while you are having fun or does it just forget about it because its unchangeable? Why is it so much more relevant in your thoughts while you are not enjoying yourself?