All of that being said, this pretty undeniably means that happiness is nonexistent and unachievable for all good and selfless people. Since, ya know, no one can be liked by everyone, anyone who does indeed care what others think can never be happy. So really, the world would be a much happier place if everyone was selfish and took care of their own happiness before anyone else's, by whatever means necessary. But that would inevitably lead to widespread murder and genocide, stealing, and other terrible practices, right? Because the powerful would take control of the oppressed to be happy, and the oppressed would destroy the powerful to be happy, and the world would degenerate into chaos and anarchy (which isn't necessarily chaotic, but would be in this case). So, we go back to the selfless definition of happiness. But no one can get everything they want if they also want everyone else to be happy. So, even if you are content with your life under this definition, you can't ever be truly happy, since you will always have an idea of all the things you want and can't have. I define happiness, personally, as the absence of a vision of, or the absence of a desire for, another side on which "the grass is always greener." (My friend Jason defines it the complete opposite way, so perhaps he can respond to this at some point and sort this all out.) Under this definition, then, you can't ever be truly happy.
I propose, then, that true happiness doesn't exist. Unless you are religious and believe in a Heaven, in which case you've beat the system. Kudos.