I think a lot of people tend to keep their problems to themselves but I don't think that is the same thing as "isolating" the way that we do when we are depressed. People may keep to themselves because they are embarrased or just think it isn't "being strong" to share their problems or whatever... but isolation goes further... we believe that others don't want to help, that we are not worthy of their help, that people don't want to be around us... That is all part of what makes "depression", well, depression... and not just "feeling sad" like so many people mistakenly think it to be.