There are people who are so unhappy in life that they do not like seeing other people happy or having what they don't have themselves, so they try to ruin it any way they possibly can. Sometimes it's anger, jealousy, immaturity, a grudge...the reasons vary but the result is the same.
They are demons.
they are miserable themselves
Weakness makes them do it
Because that's how they get their own. Its easier to destroy something than to build something