May sound corny, but I think it mostly comes down to how you feel about yourself. If you are happy with yourself, who you are, have become, life in general, you won't easily feel lonely. Sure some ppl need to socialize more than others, but if so, you can find that.
I don't need socializing / going out very often, I could easily spend weeks on my own. Have been there as well to be honest, maybe twice a week spoke briefly to the girl behind the cash-register in the supermarket and that was it. Don't bother me none. But I feel good about myself, I'm happy with where I am in life, who I am etc.
I think that really is the key to happiness, whether alone or with someone else.
I have a happy feel and look about me, so if I do go out and socialize, I have no problem with it whatsoever.
The road to finding happiness within isn't easy though. Got the T-shirts, lol

Good luck!