Not so rose-colored glasses

I always thought that if something felt good at the time you did it, it should stay good.  For example, if you had an awesome time at a concert, but your car got stolen while you were at the concert, the concert should still have been an awesome time.

Likewise, say, you don't tell someone something until after a dinner, or a party, so they have a good time there.  When you tell them later, does it erase the good time they had?

Unfortunately, it seems so.  My last 6 months in China were great... up until the 5th month or so.  Now, my entire China experience feels shitty.  When I think back on it, I know I felt really great at the time, but now I can only feel regret, or disappointment.  

It sucks that feelings are retroactive.