People who are negative and "unhappy" cannot relate to joy.
Some folks are just downright cynical about life in general.
It's possible that as a child they were disappointed one too many times and decided never to get their hopes up again. If something good happens in their life they refuse to celebrate because it's only a matter of time before something bad happens.
The only way to deal with such people is to distance yourself from them or limit whatever time you do spend with them. It's unrealistic to expect people to change unless it's something that (they) want to do.
We're entitled to choose whom we spend our time with!