This is an illness that some people just can get cured from. and there are few reasons why they can never be cured from this illness.
One is they are frustrated. Why? Because they tried to have the same life that you have but they just can't/ Poor people...
Another is that they have a sad, sad life. They can see that you have a happy life and they just cannot have that same happiness that your heart do have.
Another possible reason is that they cannot reach your level. At some point they see you as someone who is on a higher phase of life. So they try to talk at your back hoping that somehow, they could bring you down.
So with that, never stoop down to their levels. Just ignore them and thank them for appreciating the GREAT LIFE that you have :)