This is just my opinion, cats are happy when they feel secure and in control of their environment. Long ago, an outdoor cat had his/her territory, were in control of it for the most part, they were happy. Now, the outdoors in most places would be very stressful for kitties. Now, I do have to interact a lot with my boys, but they seem happy and well adjusted. As long as they have sunbeams to lie in and window ledges to sit in and look out of, they seem happy.