I'm very much an introvert and I do need a lot of alone time, and I get exhausted by too much interaction with people.
But when I was a child, teenager and young adult in my 20's, I would say I was also cripplingly shy. At one point I was pretty much a selective mute in school, though normal again at home around family. But my shyness was profound and agonizing for most of my young years, and crippled me socially because even though I was always an introvert, I did desire to have friends. I managed to have a friend but it was usually only one at a time, although that's about all I could handle anyway.
As I got older I learned to break out of my shyness and I really am no longer shy of people -- I can rise to the occasion and seem friendly and outgoing when the situation requires it. But I'm still an introvert and I endeavor to cut the requirement to be sociable out of my life as much as possible.
Recently there was so much going on socially, in the form of a huge amount of sudden, unwanted contact with acquaintances in my life, that I practically had a mini-breakdown and had to take a week off work just to carve out solitary time for myself again, and heal a bit. I'm getting worse effects as I get older, from too much social stuff, to be honest.