Here, the speaker compares a person they love to a cigarette: something that feels good in the moment but quietly does damage. The opening lines say the relationship leaves them "weak" and "numb," as if the person drains their energy and even their ability to feel. The speaker is aware of the harm, since the person is "destroy[ing] my insides," yet they still smile "like a fool." That phrase shows a mix of embarrassment and helplessness.
The last stanza turns the comparison into a habit. Lighting the person up "twice a day" suggests a routine the speaker can't or won't break, and the final line, "bitter sweet," sums up the whole feeling: pleasure and pain tangled together. The mood is resigned and a little dark. The short, plain lines feel like a confession, as though the speaker knows exactly what is happening and has chosen to accept being ruined anyway, which is what makes it unsettling.