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?

That is pretty complicated, in the more extreme examples is a combination of a personality type lacking empathy and a parapholic disorder linking lust with sadism.

Sometimes it's just because they can.

Probably because the have been treated poorly and finally snap under the pressure.

"Hit me, hit me," begged the masochist. "No!" sneered the sadist.

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.

You answered your own question. It's because they aren't happy with themselves or they are simply sick sadistic bastards, born with some bad wiring.