Tell me not, Sweet, I am unkind, That from the nunnery Of thy chaste breast, and quiet mind To war and arms I fly. (1-4)
The opening of the poem takes the form of a justification: the speaker is eager to explain his decisions and the principles of honor that inform them. While they may seem "unkind," the speaker is at pains to prove that they have a purpose. The rhyme on "fly" and "nunnery" equates his love of Lucasta with the fleeing to war in an ingenious way.
Yet this inconstancy is such, As you too shall adore; (9-10)
The speaker almost suggests that normally bad qualities—like "inconstancy"—can take on a new association when they are motivated by something as praiseworthy as honor. He tells Lucasta will "adore" his "inconstancy." Why? Because his willingness to set aside something as pure and heavenly as her is nothing short of an admirable sacrifice.
I could not love thee, Dear, so much, Loved I not honour more. (11-12)
The fundamental principle that gives the speaker's love for Lucasta meaning is his love of "honour." What's interesting, though, is that the poem portrays his love for Lucasta as a very pure, "chaste," type of love. That sounds plenty honorable, too.