How was it that he was collecting 20 million annually in "Friends" residuals and STILL unhappy?

Don't know. For what he ended up believing in and striding for, he could have set up a mega-foundation to help folks out like him.

He was probably upset that he never matched the level of fame of other actors of his generation: Tom Cruise, Keanu Reeves, Charlie Sheen, etc. He seemed to be seething with hatred and resentment.

He had problems with alcohol even when he was very young, before Friends became a hit. I also think he was a loner who kind of geeked out on stereo equipment/video games/high tech stuff. After a few days that gets very boring as you don't have anyone else who cares, or anyone else to share it with. Of course, he probably could have joined geek clubs around L.A., it would have at least got him out of the house, made it more interesting.

Ultimately, I think he got bored and tried to find the perfect high and came up short.

I guess that saying of "Money can't always buy happiness" rang true somehow with him??

Money can't buy happiness. He explained in his autobiography.

While money can free you of certain worries, it doesn't free you from them all. A lot of people with money kill themselves, and a lot OD needlessly. Happiness is finding contentment with what you have and the strength to achieve what you need to be content, which is rarely an abundance of wealth.