I doubt it was something so simple that he thought it wouldn't work. Maybe it was part of it, but depression doesn't even always have a clear cause anyway.
When I was depressed, my dad asked me well why are you depressed. I'm like dad it doesn't work that way.
But yeah not being able to see akari for so long...not connecting well with other friends where he was then (I think it seemed like that, I mean the other girl said he seemed different, and as far as I remember she was the only one we saw him talk to and he definitely wasn't really looking at her.) It could be a number of different things, and not something that's easily definable or noticeable, since I don't know that he would have thought, "oh hey I'm depressed".
Come to think of it, was depression a big topic in 2007? Maybe in japan it was...in tokyo certainly. I remember years ago, maybe around that time, there was a lot of discussion of youth suicide, so it could have more a presence in the japanese conversation in 2007 than it was in the US conversation in 2007.
Anyway, even if we claim it's depression, it might be silly to try to find a clear cause for it, other than just noticing things around him that weren't ideal for his mental health.