Yeah I was about to add a note that money is usually more of a motivator than desire for roommates. If you live with Craigslist roommates, it's probably because you couldn't afford not to. But quite a few people live with friends and enjoy it, it's pretty common to not want to live alone.

Some of it is by choice. From what I've seen, even people who can afford living on their own eventually get roommates once they realize that it's hard to meet people and make new friends post-college.

Both. The marginal cost of an additional bedroom is miniscule compared to that of a studio/1 bedroom apartment, and having people to hang around with/socialize/cook is a plus.

I would argue by financial necessity, but it depends on the region I guess.