Just this really.
I think if there's a disparity of incomes / lifestyles etc it can cause some people to be spiteful and negative towards their friends, even if it's a subconscious thing. Particularly if someone's income / situation changes for the better
I've seen it happen times. Someone starts doing better in life and some people don't like it. So the subtle bitching starts and the talking behind their back etc (I personally distance myself from people if they do this)
It's happened to me as well, I've been really broke in the past, I was a single mum, I've been days off being homeless, I've been on benefits,
I've lived in really shitty accommodation in a bad area. But in the last ten years or so I've improved my life a lot, I have a decent job, own a nice house and also had a bit of good luck which has helped. And when things started to improve for me I noticed one or two friends start to make digs at me (again I distanced myself). And I'm sure it's because I am no longer someone for them to look down on.
Anyway just something I've been pondering!