I'm a loner too. But not liking to go out and mingle with people isn't some kind of disease. Some of us just prefer to be alone. Personally, I find parties really tedious - I'd much rather spend time on my computer.
Now if you WANT to mix with people, you're not a loner, and being an extrovert who has issues being around people can be a problem.
If you don't want to go out and mingle with people, don't. But if you actually 'fear' mingling with people and if that's what's making you depressed, then you may have a social anxiety disorder.
