I used to bike commute about 7 miles each way up and down some intense hills; it was part of my routine, and I had to do it. Now that I'm working from home, sure, I could go out and ride 15 miles every day, but it becomes a matter of discipline.

Same with socializing. I tend to prefer solitude, so socializing, like exercising, now requires discipline – I have to make myself do it – whereas before it was baked in, and unavoidable.

People recommending a dog aren't wrong. I do have a dog now, and walk at least 5 miles a day. My dog is an idiot so I can't really do the dog-park thing, but if you have a non-idiotic dog, there's some socialization to be gained there, too. BUT: it's a big commitment, and will have an impact on big life decisions later on like what apartment you can rent, whether you live in a city or a suburb, how easy it is to take a vacation or travel abroad, etc., so it's not like it's a no-brainer or an easy solution.