Episode 4 or 5 where he meets with Sempai again and she says, and I quote, "If only we had properly dated back then you probably wouldn't have turned into a hikkimori."
It probably comes from personal situations but I think that guys that dont get dates during their high school years are seriously fucked up for life and that may be a major cause of hikkimoriness.
The other notable one was the episode where they went to return the goods to the class president and found out about the class presidents brother.
And finally the ending.
Theres another one too, but I forget which episode it ws.