St. Francis de Sales:

Love is bittersweet, and while we live in this world it never has a sweetness perfectly sweet, because it is not perfect, not ever purely satisfied.

George Herbert (“Bitter-Sweet”):

Ah my deare angrie Lord,
Since thou dost love, yet strike;
Cast down, yet help afford;
Sure I will do the like.

I will complain, yet praise;
I will bewail, approve:
And all my sowre-sweet dayes
I will lament, and love.