All good things...

People say "nothing lasts forever" and "all good things must come to an end". I really don't like that notion. Why? Why do good things have to end? Obviously we are all going to die so everything, at some point will end. Five billion years from now, the Sun will go all red giant and expand out passed the orbit of the earth, and this little blue planet will be no more. Our solar system, at that point, will be at the precipice of the end. But more immediately, why? Why can't good things last? If two people are enjoying each other's company, and the time they spend together, why does it have to end? Because invariably people will always be enticed by "the other side". The what-ifs. The prospective notion that there is the possibility that you will find more happiness than what you currently have. What is so attractive about that notion? Are people so discontent with what they have, that they always want more, regardless of whether they need it or that unnamed thing is better than what they currently have? Why can't human beings be happy with what they have? Would the world immediately end if we just accepted our current state as being just right? I don't have any answers, all I know is that right now I'm in a shitty mood and the notion that if I do end up being happy, at some point, this idiom ensures that it won't last. That pisses me off, so what's the point then?