In the opening lines of the first stanza, the speaker asks why he and his wife should stay together when they are no longer in love. He implies that a marriage vow made long ago is not a good enough reason. In the opening line he calls the marriage vow that he and his wife made to one another "foolish." He thinks it is "foolish" because emotions change, and therefore it is "foolish" to promise that you will always love the same person as much as you do when you marry them. In the second line of the poem the speaker emphasizes this idea that emotions change when he says that the marriage vow was made "long ago." The implication here is that a lot of time has passed since the vow, and thus it no longer has much meaning. In the third and fourth lines of the poem the speaker says that the passion that once existed between him and his wife has "decay'd." This again emphasizes the point that, from the speaker's perspective, it is "foolish" to expect love to endure forever.
In the second half of the first stanza, beginning with line five, the speaker says that he and his wife have tried to love one another. He says, "We lov'd, and we lov'd, as long as we could." The repetition of "we lov'd" suggests that the love between the speaker and his wife has not died for lack of effort. In the sixth line the speaker says that eventually he and his wife were simply "lov'd out," meaning that they each ran out of love for the other. In the seventh line, the speaker proclaims definitively that "our marriage is dead," indicating that there is no way for him and his wife to rekindle their love. In the last line of the first stanza the speaker emphasizes that there is no longer any "pleasure" in their relationship.
In the second stanza, the speaker tries to prove how "foolish" the marriage vow is by comparing a marriage to a friendship. In the opening two lines of the second stanza the speaker suggests a scenario in which he has "pleasures for a friend," but "farther love in store." In other words, the speaker asks the listener to imagine a friendship that the speaker has and which the speaker finds pleasurable, but which fades away when the speaker meets somebody else that he loves. In the third and fourth lines, the speaker asks why it would be wrong for the person who found love to, at that point, commit less to the friendship. The implied answer to this rhetorical question is that that person has done no wrong at all.
In the second half of the second stanza, beginning with line five, the speaker says that it is "a madness" for one friend to be "jealous" or possessive of another who finds love elsewhere. In line six of the second stanza, the speaker says that it would be equally irrational of the friend who found love to insist that the other friend not replace him with somebody else. Here, the speaker is implying of course that it is just as irrational of a wife to demand the same of her husband. In the seventh and eight lines of stanza two the speaker says that all that can come from such jealousy and possessiveness is "pain." In other words, when a man and wife stay together simply because they feel obliged to honor their marriage vow, they will only cause each other pain, which neither of them will be able to "hinder," or prevent.