One of the sad facts of life

How often do you notice people? I mean - talk to them, ask them how they're doing? And how well do you know your coworkers, what's going on in their lives. Are we even supposed to know everybody who we get in contact with?
I had a coworker. I knew his name, the area he worked at and the fact that he liked fishing. For as long as I worked in the company, less than 3 years, occasionally, I would see him doing homework from this math textbook. That's pretty much it.
He shot himself today. People that were closer to him say it wasn't an accident either. Almost all of them said he was a really nice guy. Very polite. Kind. Suicidal.
For all of this time, I've talked only once to this guy.
Never noticed him.
Sad.

Why?

I always wanted to know - why do people do what they do? Why did I even open this blog? I opened different ones before, and never kept it up.