What could have happened to my old friendship?

I used to be best friends with this guy when I lived in California. We were friends since we were little kids. I moved away during 6th grade, but we still got to hang out occasionally until I moved away for high school. The last time we saw each other was when I was 14.

It has now been over 10 years since we’ve spoken. He finally showed up on Facebook a few weeks ago and I’ve tried to contact him, but he’s unresponsive. I haven’t heard a word from him despite messages, wall posts, photo comments etc. I’ve seen that he’s been active because he’ll occasionally post photos or talk with others. He even added himself as a fan of my art.

So my question is: what the hell happened? We never had a falling out or argument. We just lost touch. It looks like we live fairly different lifestyles, but I can’t imagine that has impacted his decision to drop a note. I wouldn’t be so persistent if I wasn’t so confused.

So, any insight into the possibilities of the breakdown of our friendship would be most welcomed.