Your Loneliness is Your Soul's Secret
As far as most people can tell, you are as social as you want to be. You seem content with the amount of time you spend with others.
You seem busy, if anything, and people assume you can't be bothered to hang out with them. No one knows that you're just being lonely instead.
You hide your loneliness for a variety of reasons. You are embarrassed about how anti-social you've become.
It's also possible that you don't know how lonely you are. You may be hiding your loneliness from yourself.
If your loneliness was less of a secret, you might be surprised how many social opportunities open up for you.
It's also likely you'd be happier if you came clean about your happiness. You'd see you aren't alone... in fact, many people are as lonely as you are.