Since we know "A Valediction: Forbidding Mourning" was written for Donne's wife Ann, it's not a stretch to call this a love poem. Most love poems, though, focus primarily on the beloved. In this case, the speaker spends most of his time defining the nature of the love they share. The final argument is that real love is powerful, unconquerable. Real love can't be defeated by distance and real love doesn't fall apart at the thought of being apart. In other words, "Love lifts us up where we belong…"
Donne argues that people who cry and mourn when they have to be away from their lover aren't really in love. Love's bigger than that (so knock off the waterworks already).
All this argumentation about love falls right in line with Donne's time period. All the writers boasted about how their love was better than everyone else's. Don't believe us? Check out Shakespeare's "Sonnet 130" as a classic example.
Donne wrote some pretty saucy poems, but for all his playboy bravado, he stayed married to the same woman until her death. As a religious man, he cherished faithfulness to one's spouse. "A Valediction: Forbidding Mourning" is a love poem, but it's also a poem of praise to his wife's faithfulness. It is her faithful, enduring love that holds their relationship together, seeing as how, you know, he's not letting the shirt touch his back in his mad rush out the door.
Donne emphasizes his wife's loyalty, but he also is subtly requiring her to do nothing but stay at home and pine for him until he comes back. Good times for her.
The poem takes an extremely high view of marriage. Their love is committed for better or for worse. (We're guessing that it's about to get worse, for the wife anyway.)
"A Valediction: Forbidding Mourning" is a love poem, but Donne needs a whipping boy to compare to his all-powerful love. He uses lust. He kicks lust all over England in this poem as a way of showing just how much better his love is. Poor lust. It never stood a chance.
Lust is the opposite of love, if you think about it. Lust wants to take; love wants to give. Lust is skin deep, but love is body, mind, and soul. Donne calls our attention to the simple truth: when two people who are "in love" can't stand to be apart even for a little while, doesn't that tell us all we need to know about what their love is based on?
Donne is overstating his case here and it's a little ridiculous. I mean, are we supposed to discredit anyone's relationship simply because they cry when they have to be apart?
Donne overstates his case for good reason—he believes no one could have a love like his. Compared to his love, everything else is just shallow lust. So there.
Body and soul, heaven and earth—Donne was obsessed with this central contrast between the life we have to live in this goofy body (I mean, we have to put on deodorant just to keep from smelling funky), yet we feel ourselves endowed with a willful, strong, living spirit. This contrast kept Donne occupied for the better part of his career, as he bounced between poems suitable for the bedroom and poems for his pulpit. In "A Valediction: Forbidding Mourning," Donne contrasts love that is based on the body, with a spiritual love that is able to transcend the flesh and is based on the soul.
Hop on the therapy couch, pal. Donne writes about spiritual love in this way because he is at least partially afraid of his physical body. His more promiscuous early life and writing makes him feel dirty and guilty, and so he praises anything spiritual and speaks poorly about the body.
Of course Donne has to make the argument that spirits matter more than bodies. He is about to be away from his wife's body for months. He's just trying to make himself feel better. (Think it's working?)