Even as an introvert it's always people. After college, before getting married my (now) wife and I had the same problem - but we noticed it wasn't just us: everyone we knew felt lonely. We decided to start getting together with all our friends and acquaintances almost every night after work. We hosted or set it up for about two weeks before it took on a life of its own and we didn't have to be present for each night;…
1. I don't feel like I have enough energy to do things after work, and I find myself unable to socialize for long periods of time.
2. My friends, or at least friends I know in the real world, are running into number 1, and often have different work schedules anyways.
Modern society feels isolating. When I try to be social, I find many people are disinterested. I myself hardly have any energy to be social even though I feel like it would be good for me. Perhaps general health has something to do with this too, but it sure isn't convenient.