Any advice on this situation?

Alright so this whole scenario came to be because when I was 16, I'd merely glanced over at this guy (20m) for a couple of seconds from across the room. He because too caught up with what I did, and when he saw me a few months later, he began continuously staring at me from afar. The next time I saw him was a year later. He froze in his tracks (shocked) when he saw me for the first time after a year. He then began standing directly behind my chair to the point where I couldn't pull my chair back and kept staring from afar. At the time, I became interested in him, but soon dropped it as he wouldn't approach me and wouldn't reply even when I tried talking.

However, it didn't stop. During events, he'd hide behind a wall or door and stare from a distance. Over time, his staring from afar became prolonged to the point where he began watching me from afar. Sometimes he kept leaning over to watch me even when I turn the corner to avoid him. Long story short, I only see him occasionally but this whole thing continued for almost 7 years before I finally realised that this was stalkerish behaviour. I felt nauseous, angry, weak and I started breaking down. I decided to avoid him, and would purposely not attend events because I can't with this, I feel trapped by this.

What is wrong with him? What is causing this in him? I've spoken to several people (I have a lot of mutual friends with him) but he only does this to me.
Any advice on this situation?
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