Writing in his autobiography, John Stuart Mill reflects on the fleeting sense of happiness:

I had to ask myself, "Suppose that all your objects in life were realised; that all the changes in institutions and opinions which you are looking forward to, could be completely effected at this very instant: would this be a great joy and happiness to you?" And an irrepressible self-conscious distinctly answered, "No!" At this my heart sank within me: the whole foundation on which my life was constructed fell down. All my happiness was to have been found in the continual pursuit of this end. The end had ceased to charm, and how could there ever again be any interest in the means? I seemed to have nothing left to live for.

 In an early scene from The Shawshank Redemption, Brooks Halen is released from prison after being locked up for fifty years. He then hangs himself, unable to cope with life on the outside.

Although these examples are a bit dramatic, I see less dramatic versions of it all the time. There is little motivation when the chase is taken out of the equation, when there is nothing to go after, nothing to challenge you.

Just as important as hitting your goal is identifying what's next.