sweetness tinged

...and at the
moment, some strange melodious bird took up its song, and sang,
not an ordinary bird-song, with constant repetitions of the same
melody, but what sounded like a continuous strain, in which one
thought was expressed, deepening in intensity as evolved in
progress. It sounded like a welcome already overshadowed with
the coming farewell. As in all sweetest music, a tinge of sadness was in every note. Nor do we know how much of the
pleasures even of life we owe to the intermingled sorrows. Joy
cannot unfold the deepest truths, although deepest truth must be
deepest joy.

-- George MacDonald, in Phantastes, a Faerie Romance