Wow, OP. Your post actually blew my mind because I am getting to be the exact same way. Except I have a girlfriend and occasionally enjoy to go out, but definitely not as much as before. I'm not on any meds though. I used to be opposite, going out every weekend, always being with people.
So I do feel like you do. And I am content about it. I'd rather be with my girlfriend or alone using which ever substance or browsing the internet for entertainment and gathering so much knowledge. Or all 3! haha.
I have been looking into this and a lot of similar reports point to depression. For my case anyway. They say a sign is when you'd rather be alone than contributing to society/being social. So if you relate to that or do some research on depression in relation to the points you make here, then that might be it. Otherwise, what I've been thinking, is just severe pessimism. I'm extremely pessimistic. And you saying "what's the point?" to friends is a sign of that too. But I'm sure even that falls under depression somehow lol, I haven't researched that aspect yet.
If that's not the case then maybe you just need to find what you do enjoy (it's difficult, I enjoy nothing at this point). Or force yourself to be in certain situations because you never know what positive things may come from them.
Oh, right, it probably does correlate to substance abuse. None of my friends are really into some shit that I do so I'm really only left with myself or my girlfriend, so that adds to being away from people. Maybe it's just finding new friends. Either people you can bond with over drugs (but not enablers causing a serious problem, just occasional fun), or that you have more in common with in general and can have fun with to decrease the alone time and drug use.
God I talk a lot.
EDIT: Also, how old are you, OP? I'm 20, so what adds to this mentality is the fact that I've done what's "fun" so much that I'm bored of it. Parties, sooooooo much in high school. Bars/clubs, soooooo much over the past 2 years. Girls, have had my share and am in a relationship now. So, maybe now it's just a "finding yourself" phase where you build the independence necessary to continue into the future with success. If you're older than 25 though then, rule this theory out, haha.