Nikky's poem begins like a standard declaration of devotion. The first two lines promise love and care that no one else can match, and a reader might expect a tender ending. Then comes the pause, "And last of all...", which builds a little suspense before the punchline: no one will put up with your shit as well as the speaker does.
The joke works because it's also the most honest line. Love and care sound nice, but patience with someone's worst habits and moods is what long-term closeness often looks like. The speaker admits the other person is difficult at times, and claims to stay anyway. The tone is teasing and affectionate, and the final "Do..." trails off as if the speaker is smiling or shrugging. The mood is warm, playful and a bit self-aware. It suggests the speaker knows that being loved fully means being loved flaws and all.