Why do I feel so empty & upset when a show I like ends even if it started suck?

Exactly!!! I always think the same thing!! I sat in the theater, right to the very end of the last Harry Potter movie, through every last credit, to the point the lights came on in the theater, just HOPING there would be a tiny teaser to something more. That was such a sad night. I did not want it to end. I think it is because so much of your emotion is invested in these characters, and they actually do end up feeling like a friend you have to say goodbye to.

Just exactly for the reason you said.  We get to know the characters and they become like old friends.

I often just play it from the beginning but don’t watch it. Just have it playing in the corner of a room with the sound low while I’m doing something else

you can buy the series on dvd and watch it down the track