I believe some people like to keep others down because of their own insecurity. They feel better when they think they are better than you. When you start encroaching on success, that scares them because then they need to move forward too. It is easier to say something negative to you than to do the work necessary for them to move themselves ahead.
It's true that it can be easier to focus on the negative rather than push yourself to improve. Which is a shame because you can only gain by pushing yourself forward. You'll waste time by focusing too much on what other people are doing.
Isn't that the truth! Unfortunately, many have still not figured that out yet.