Pushing Ice by Alastair Reynolds is one of my favorite scifi books and it deals with this question in an interesting way. It proposes that Time is the great filter. Life exists in this galaxy, but intelligent life is so fleeting when considering galactic distances that the probability of one sentient lifeform finding another during their "peaks" is vanishingly small. Extinction, societal collapse, evolution to a higher form, whatever you want to imagine, it all gets in the way of the fantasy of meeting a thinking being from another planet.