I have found that people become uncomfortable (to varying degrees) when their friends(siblings, family, SO's) have greater success over time than they do. This could mean a bigger house, better car, promotions, etc.. They love/like you when you are down or not doing as well as them but, will they stay and be supportive as you move ahead of them?
Thus, a truism:
A real friend will be supportive as you move ahead on the path to success and still be there when you have "made it" and they have not.
It is rare to find people in and throughout our lives that have that quality...who are secure within themselves. So that where you are in relation to themselves is not a factor in friendship.