One part of it is the fact that when you're alone you're more likely to drift into unhealthy habits. Being isolated means that there's nobody to judge your for staying in bed, eating junk food and scrolling reddit all weekend. It's similar to how I'm more likely to keep my flat clean if I have people coming over, or more likely to get my work done if I'm in the office surrounded by people, or to set more ambitious goals in life if I'm surrounded by more successful people. Not everyone is motivated that way but I find a bit of judgement from other people to be a healthy thing, or at least it's something that pushes me in the right direction. Being too isolated for too long causes your life to rot in a way.