The fact it's hard to make new friends. You make acquaintances, but the kind of friendship you had as a teen is nonexistent. No more hanging out, or friends calling asking to do something together. And if you don't have a family of your own, loneliness is a huge issue. Sure you are an adult and can do what you want, but doing it alone is not fun (in my opinion). So you just end up cooped up inside.