Because even if you achieve happiness there will be something that will bother you. It is in our human nature to look more at the bad sides than the good sides.

Money: a billionaire stills has problems avoiding taxes and losing all assets.

Early-pension: people need a purpose to do something, otherwise you will get bored.

One happy family: outside influences (boyfriend/girlfriend) can negatively influence you.

However it’s not all bad, we just have to focus less on the negative sides and look to what we can achieve.