People are prone to be let down by displeased ending than the whole happy experience. Every time an unhappy event happens at the end of a cheerful experience, every good memory of the whole experience you've achieved get ruined in everything. Despite the whole vivid, beautiful memory you experienced, you forever stuck in your bad memory. You frequently encounter disordered, or depressed feeling.
And something is memorable not just because it's been good, sometimes it's been very bad.