People is, I think, it's their nature - some people's nature, in a way, to be angry or jealous or just spiteful about somebody else's blessings.
This quote by Jill Scott is referring to people who are spiteful, jealous and angry about other people success or blessings. It is nearly impossible to find people who will happy for your success. Sometimes, even our family, spouse, and close friends will be angry about our blessings and success. This is something we have no control over and cannot change but must accept.
It might be tempting to share personal achievements with our family and close friends, however, it is not wise to do so. People might pretend to be proud, but deep inside, they are angry, jealous and/ or plotting ways to destroy or steal your happiness or success.