Does it ever explain it? Apologies if it does.
He says something along the lines of, "My reason for wanting to kill myself is different from others'." And then he lists a few reasons which don't apply to him, one of them being "hopelessness". I would class hopelessness in with general depression, so it can't have been that.
Could it have been guilt over his possible murky past in the Yakuza? Any other ideas?
"Could it have been guilt over his possible murky past in the Yakuza?"
Probably that and he couldn't have known how to start a peaceful happy life.
After his half assed suicide attempts you should notice he does not really want to die
Its more a conceptual thing about what death entails
He talks about how death in books is perceived to be relaxing and so that is why his suicide note says "This is bliss." because death is blissful and relaxing.
Pretty sure it was something like that.
Film of 2013
http://www.imdb.com/list/MyXTL3f7TtQ/