Three short lines, and a lot is left unsaid. The speaker is talking to someone, a "you," and hoping that this person doesn't fall for "her." We never learn who she is, or why falling for her would be a problem. That gap is what makes the poem work.
The opening, "for better or for worse," borrows the language of wedding vows, which gives the lines a quiet irony. Marriage promises to accept both good and bad, but here the speaker only seems to expect the bad. "Let's just hope" sounds casual, almost shrugging, yet it also sounds like someone who is bracing themselves.
Judging by the tone, the speaker may be worried, jealous, or hurt, and perhaps a mix of all three. They might be protecting the listener, or protecting themselves. The mood is tense and a little weary, a feeling held back behind an offhand voice. Because the poem is so short, the unspoken part lingers after the last line.