The speaker is hurting over a person they love, who seems happier and different when talking to someone else. The refrain "it's killing me slowly" repeats like a dull ache that never stops, and each time it returns, the speaker names a new detail: the thought of the two together, the way the person laughs, the sudden shift in mood. What stings most is the comparison. The laugh is something the speaker never got to hear, and the person seems to "become someone else" around her.
The tone is quiet, raw and unpolished, as if the speaker is thinking out loud. There is jealousy here, but also confusion: "I can't figure out why" suggests the speaker keeps searching for an answer. The ending, with its hesitant line break after "maybe," feels like a reluctant surrender. The speaker half-accepts that the relationship was "doomed to die," though the word "maybe" suggests the speaker is not fully convinced and is still hurting.