What causes some people to be Sadistic? Why do such people enjoy causing pain to others? Is it because they aren't happy with themselves?

Their brains are literally wired that way from birth. If nature can make you empathic it can also reverse that connection and make you a sadist. There are even signs that do show up in childhood. Lack of empathy, charisma, hurting animals for fun, etc... So it's something a person is born with generally.
Although life circumstances can make one sadistic as well but in those cases they aren't true sadists and could be acting as a tit for tat kind of thing or feeling weak and powerless.