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 (from Phantastes).
The paradox of today's pleasure already intermingled with tomorrow's sorrow... In every anticipated moment of joy in this life, there is already a seed of sadness sown. Maybe the connection of joy to sorrow is simply God's way of reminding us of our eternal home, where He will wipe away every tear in that endless day (Rev. 21).