Disappointment occurs when you realize something that you thought you wanted does not satisfy you. Disappointment builds up over time. Chasing after something you never quite catch up to can be oddly gratifying. The feeling of momentum, forward progress towards some desire, makes up the meat of human experience. Chasing a goal is a kind of purgatory on earth, a suspended state between wanting and having. When some process or another is repeated the requisite number of times (which strikes me as a dishearteningly accurate description of most human activity) one will sooner or later find themselves at the point which they desired to be. The time has come to rise out of purgatory and go to either heaven or hell. Do not take this analogy too seriously. Neither the elation which comes with achievement nor the sinking feeling of disappointment is eternal. Most of the time one quickly reorients themselves, refocusing on a new distant but visible point on the horizon to begin slowly working their way towards.
Sometimes this doesn't happen. This sinister potential lurks within both disappointment and achievement. Both exist outside the network of values and meaning (which ultimately coalesce into desires and aversions) which the mind requires to exist. Disappointment brings with it a decidedly nihilistic sentiment. Something you placed so much of yourself into, upon which you pinned your hopes and to which you sacrificed untold hours of your life, turns out to be nothing special. So what now? Try again? Achievement can seem superficially more satisfying. It brings with it an obvious feeling of accomplishment, of self worth, a certain lightness of the soul. This lightness, however can turn sour. I got what I want... so what do I want? It can gradually fade to the same place as disappointment. Of course, achievement more often than not breeds the impetus towards further achievement. Most of the time, but not always.
Disappointment need not be fatal. But it builds up over time. It gets harder and harder to set goals, to work towards some objective, to care about whether something happens or doesn't happen. I am not there yet. I still see so much wrong with my life and my behavior, so much room for improvement that I can't help but try. But I have been disappointed enough to be scared. Scared that one day I will wake up and find myself in a world of stone and plastic and dust and dead flies on my windowsill and I won't care at all.