I had just the most outrageous crush on someone at work for like a year. Then, I woke up one morning, and it was just gone. I was over it, like that, and it stayed that way. How does that happen??
It ends the same way it begins, with a little neurochemistry and timing. Also, cognitively speaking, a crush starts out as raw emotion, then becomes a set of feelings, which then develop into thoughts. If those thoughts don’t develop into actions, eventually the underlying emotion fades, and once you’re done thinking about it, the crush is over. You wake up one morning and the feelings are gone.
On a side note, if you understand the cognitive/emotional process and have some control over your emotions (easier said than done), then with a little self-discipline, you can effectively end inconvenient crushes on your own schedule.