There are many great quotes that say "I miss you" better than those three words alone. The words "I miss you" connote the feeling of loss that comes to a person when a loved one is absent, no matter whether the separation is temporary or permanent.
Lines from "Separation," a poem by American poet W.S. Merwin, sum up missing someone well: "Your absence has gone through me/Like thread through a needle/Everything I do is stitched with its color." When someone misses another person, sometimes one feels like it affects "everything [they] do."
Alcibiades, a statesman who lived during the Peloponnesian War, said, "As contraries are known by contraries, so is the delight of presence best known by the torments of absence." Alcibiades' point is that one cannot know how much one delights in the presence of a loved one until one has felt the sting of the loved one's absence.