Is it normal that my "fun" is gained by spying on other people's fun?

I do that too. It really makes me smile to see people happy, also really makes me sad when someone looks sad. You're probably just an introvert, which is why you don't feel the urge to join in (just like me). I do have a few friends, but I don't see them often. Because I don't want/need to. I suggest you do some research on being introvert. Don't let others tell you do what to do. Do your own thing and don't care about what others think of you. However, do indeed make sure that you don't get mistaken as creepy. If you want you can contact me for more about this.

Lots of people get happy by seeing other happy people. That's normal. But the fact that you have no desire whatsoever to join in.... That's a bit abnormal, but not uncommon.

Do you think maybe you have social anxiety disorder?

You're A stalker U.O.E.N.O. it.

oooh, vicarious fun is great fun but i feel being a hidden shadow is kind of creepy. it can be done right next to them and them not having a clue you're taking part

I don't think this is normal. I would flip out at someone if I caught them spying on me even if I had nothing to hide.
I'm guessing your empathic because you feel other peoples emotions. You should try to interact with them and maybe you'll realize you like doing that better.

How about doing something recreational that involves cooperation, let alone talking to others? I'm sure you'll make great friends with people rather than being known as the local Peeping Tom.

You realise that your behaviour could/can get you labeled as a stalker, child molester, etc.

Nope you are a stalker.

Lol, this made me all warm and fuzzy inside :).

People watching is good times :)

I do the same thing, but only when they're naked.