Ok towards the end of the film, Sam is talking to Jed/Jett outside and yells "I cant F@#K HIM!"
Why couldnt he? His video diary really didnt explain this other than he was in love with Jackson.
Habataitara modorenai to itteshare
Mezashi-ta no wa aoi aoi ano sora
I do think Sam couldn't because he was in love with Jsckson and he knew Jackson didn't feel the same. He thought it would ruin the friendship and he ended his life because just the friendship wasn't enough.
I found it somewhat cruel that Sam send Jackson the tape with you're my best friend and I'm in love with you and you're not with me and so I topped myself. It's like he blamed Jackson in some sort of way... even if he probably didn't.
Love the film though.