The best things are both known and unknowable, like your cat, your significant other, or the ocean. Here's the Boston Globe's chief art writer Sebastian Smee on Titian's "Portrait of Ranuccio Farnese," painted in 1542:
Great portraits call out to us in ways that landscapes, for instance, do not. Simply, they show fellow human beings. And so it’s natural that we project onto them “fellow feeling.” We may look at Ranuccio Farnese, for instance, and think of our brother, our son, even of our own 12-year-old selves. But what gives the waves of fellow feeling between us and this portrait their electrical charge is, I think, something beyond projections, beyond explanations; something ungraspable: the tender untouchable pride of a face like a distant planet.