I just don't get it

Man why can't people genuinly be happy for anyone.  I know I am guilty of this in the past, but I have been changing my ways.  You know this is why I normally am a quiet person, because people are so negative.  I don't want negative people in my life.    Are people just so into themselves or just really negative that they can't be genuinely happy for someone else???  Isn't it kind of a downer when you are soooo excited about something and you can't wait to share it with other people and they either dismiss it or have to be negative about it.  I mean wtf!!