The bittersweet feelings of longing and nostalgia that seem to grasp hold of us as we gaze upon a sunset, or trees in bloom are truly defined by C. S. Lewis in this passage from The Weight of Glory...

The sense that in this universe we are treated as strangers, the longing to be acknowledged, to meet with some response, to bridge some chasm that yawns between us and reality, is part of our inconsolable secret.