Love; not so ever-lasting as they believe

After a Sex and The City series marathon i finally realised; all that there is out there, is friendship. The only ever-lasting feeling. If you all reflect for a moment and think about the past and experiences you know that other people have, love comes and goes.
So I cannot help but wonder: why we all carve so much a feeling that eventually will fade away ? Why we kill ourselves trying to achieve something that it will go away after a certain time ? We live in this world of everything being fast paced, faceless and only reaching acqaintances, so why we keep trying ? Can't we just settle for what is given ? And even if love gives us the opportunity to feel full of energy and offers us a cloud of infinate possibilities for a certain time, why do we do that to ourselves? Why tortur ourselves with a sneak peak of heaven, if we will never have the opportunity to join ?