There are many reasons. There have been times in my life I have chosen to isolate myself. I had a lot of social anxiety in my adolescence and twenties. So I didn't develop any relationships. Now since I don't really have any friends it becomes very difficult to meet anybody since many of the people we may meet are friends of friends. Another reason why I remain isolated is that i have spent a huge chunk of my life in depression. The negative affect depression produces does not help create a good first impression.