Have you ever made up an excuse to avoid a social obligation so you could spend the evening by yourself? Do your friends and/or family understand when you need some quality time alone?
Uh, I do that a lot, actually. I'm just a hermit. I do like getting together with friends and all, but I do make excuses to avoid socializing with non-friends, acquaintances I feel so-so about, coworkers, etc. If I don't have a good reason, I tell people I have a lot of laundry to do. Even if I just sit around gaming all night, it's still the truth, because I always have a lot of dirty clothes that need to be washed, whether or not I actually wash them.
My friends and most of my family are usually good about leaving me alone. My friends understand (most of them are introverts too), and my parents know that I get cranky when I've been around people too long. My sister, on the other hand, has never understood the concept of "leave me alone" and consequently has never understood why I find her annoying. My little cousins can be overwhelming too. When we were in NC for Thanksgiving, Kara was being a total cling-on, even late at night when she was supposed to be sleeping. I was staying in her room (she was on the basement couch), and she came in one evening and asked what I was doing. I said I was enjoying my alone time, and she said she was doing the same, then proceeded to sit next to me and talk to me anyway. Ah well.
I'm like an electronic device. My social batteries only last so long. Then I start sending out warnings (instead of beeping or flashing, I tend to withdraw and suddenly act grouchy) before shutting down completely. My social batteries need time to recharge before I can be social again. I figure that for every hour I spend with people, I need two hours to recharge. My social tolerance level depends on the company. There are only a few people I can really tolerate for long periods of time. I even get tired of kitty attention if it's constant and demanding for a long time. (I only really get annoyed with Galahad though, because he's the attention whore and wants to be in my face constantly, rather than just being content to sit in the same room with an occasional lap session.) But I spend a lot of time at home with no other humanoids around, so it works pretty well for me.