Well, it could be that, but of course there are some people who are extremely uncomfortable around others and therefore are happiest being inside and/or alone.

No.

I can't say I know anything really about Japanese cultural pressure. But it isn't some stupid excuse to sit on your ass and fap all day. Generally one in this situation goes to the internet as a vital source of much needed social interaction. Granted it can become an enabler. But it isn't the initial excuse unless the person or character in question is a huge lazy bastard. HUGE.

It isn't a "disease" either. It's more like a disorder.

And I wouldn't put it as "happier alone." I'd phrase it as "less mortally terrified."