I isolate for several reasons.
First off, I'm an incredibly slow riser. Even when I was much more outgoing (read: younger), I required a minimum of two hours between the time I rolled out of bed and the time I felt confident about actually doing anything constructive or interacting in a positive way with others. Health issues can now stretch that time out to encompass an entire afternoon. I work nights, so I'm never up before 1:00 and am incapable of setting any kind of appointment before 3:00. preferably later.
Second, I live with chronic pain due to cervical disc disease (arthritis in the neck). It takes a great deal of energy and focus (along with Ibuprofen) for me to overcome this and be the happy-go-lucky bon vivant people expect. When I cannot overcome the pain, I can get extremely cranky and prefer to withdraw.
Third, I've always made myself excellent company and deeply appreciate solitary activities like reading. Writing is, by its nature, an intensely solitary activity and I write a great deal.
Fourth, I work in an intensely social situation (a gay bar) five or six nights per week, so I get all my socializing down there. If it weren't for my job I'd be a complete hermit.