It is in human nature to never be satisfied. The emotions that one person will experience over a period has highs and lows, between which lay some baseline that they would call feeling okay. Graphed it would look much like the period of a sine function. If that baseline of that wave function were to be zero "happy" would be positive, and "sad" would be negative.
No matter where you move this baseline people will not always be happy or sad. From their perception, the baseline is always zero, below it that person will think that things could be better, above it they will think things are going well. However, happiness is an internal selfish emotion, the type of thing only the individual can control. People can check to see if everything is working how their selfish world has idealized it and decide to be happy or sad about this. They can choose to be just as happy in the low of a trough as the peak of a crest.
That is why it is written "and the pursuit of happiness". No one can make you realize you can choose to be happy. Many extremely successful people spend their entire life in pursuit of the peak of that crest. They can see in the distance as they constantly raise their baseline, never satisfied, never reaching their peak. Then one day they catch a break and finally can rest in peace, but they lost the pursuit.
To others, this person's happy graph may appear to be y =mx+b, while to the person it felt like y = -mx+b. Some people never realize you can be happy standing on a peak or down in a trough. They constantly raise their baseline and keep happy all about them. They keep it selfish. They never learn that when things are not going how they want they can be happy for the people it is. Everyone must teach themselves how to stand on the peak of their highest crest and stay there. No one can give that to anyone. It doesn’t matter if someone loves them and all they want is for that person to be happy. Happiness can be shared but not given. It is a learned choice and I do not know the word for it which says a lot to the human condition on happiness. It is not compassion. Compassion is about feeling sorry for the less fortunate. The word I am looking for is feeling happy because you can see others that are doing good. It is a form of empathy, but I want something more specific. If someone knows that word, please share it with me.

@tarazkp I don’t mean to leave essays in response to your posts. You tend to cover topics that I spend much time rolling around in my head. I guess I’m too shy to drop it as my own post to see what people think, but a response is different. It is like, “oh hey me too”. Then at that point, if I posted on my blog I’d be stealing, so I leave a response to your bravery to speak up. I suck at twitter as you can see. Thanks for sharing.