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.