I think it's a bit of both, well, many factors. At least for me, I've both been a nuisance to some friends when I was down, but sadly have left behind some friends who were often down as it indeed stressed me and I realized I was happier overall by myself. I think it's partly psychology, partly maybe evolutionary, partly due to the superficiality and instant gratification the social media saturated times many of us live in these days have on us, making us less tolerant to stressful and non-rewarding relations and more likely to shut people out due to lack of accountability online at least. Offline, I am not so sure as I have not had a close friend since I felt left behind by a close friend when he got closer and ended up liking a friend who rubbed me the wrong way. However, looking back I realized that my somewhat violent outbursts and extremely jealous behavior would have made me leave me behind. I suspect that's partly because I grew up an only child and never got a good idea how to share the spotlight, how to be second place in someone's life, but part of that is just my personal experiences and perhaps biology, and to a lesser degree I suppose my choices. But yes, the older I get, the more detached I become from some friends who I decided I simply do not mesh with, and some have done the same to me. But it still feels wrong and I feel guilty over it most days and worry about those friends I either know or suspect I upset.