Why did my ex break up with me?

I was with him for about 7-8 months and then poof, one day he tells me half in tears that he can't be with me anymore because he doesn't know who he is. He then starts acting emo and avoids me at all costs. I never did anything to hurt him, all I did was love him and I was his first girlfriend. Fyi, he was 15 at the time and I was a year old, 16. I know, very young.

He then isolated himself for 12 months until he started talking to others again. Now, 5 years later, he has a girlfriend for 4 months but he asked me on aim, "how do you feel about me?" and i replied, "what?" and then he asked "do you like me?" so i answered truthfully and told him, "even though I have had the same boyfriend now for the past 4 years, I've never stopped thinking about you. I never stopped loving you and I know I will never love anyone as much as I loved you."

so he said "I thought so." then he told me that he liked me too..

So I'm very confused right now, but I also have no idea why he left me in the first place.

My friends think he was just very depressed (his dog he had for 12 years died 3 months into my relationship with him )and so, he wasn't able to hold me as a girlfriend because he was too depressed, but he smiled at me always. He's also very shy in person, but he's slightly better now.

I'm just confused. Oh and when I asked him will this feeling last, he said "it probably will, but I can't help it. Just make sure you tell me to stop if I ever try to kiss or hug you."

Why did he bother to tell me that??? This is a nerdy, shy guy. He is absolutely 100% not a player fyi and now a week later, he hasn't mentioned he likes me again but he smiles at me in the way he did when we met.

We used to sit on my roof and watch the stars when I was with him and then he'd bring me inside when I was about to fall asleep, lay me on my bed and then ride his bike back to his town about 10 minutes away.

Anyway, what do you think is going on?