> Who said that just because you don't have a social life you must be miserable or depressed?

Well it depends on how one defines social life. But historically a lone human is a dead human. Depression does have an evolutionary advantage in that you are less likely to approach potentially violent strangers. The lowered energy levels make you sleep longer and reduces your visibility. The misery is probably just a motivating force so that you seek out your tribe.

People in solitary confinement go crazy, quite literally. Humans need contact with other humans - that however does not mean wild parties every evening.

Happy loners tend to have few friends/colleagues/family they talk with. That is not nearly the same as no social life at all.

>who said that being alone = depression

As a loner, I will say that.

http://psycnet.apa.org/record/2006-03906-014

Do you not have many close relationships with people or do you just choose not to spend too much time on social activities like the author did?

Both

Same here.

Worked out perfectly for a while but crushed me eventually.

Not a path I want to go through ever again.

Some people might be cut out to be hermits, I am not.