Someone says the relationship is over, then kisses the speaker goodbye, or hello, the very next morning. That contradiction is what the speaker can't get past. "I'll never understand" sounds like someone replaying the moment again and again, looking for the logic in it and finding none.
The second part shifts tone. The speaker admits the other person only truly meant it a week later, and the phrase "I guess" makes it sound like they're shrugging, trying to stay casual. But the last words, "it ripped me apart," drop that act completely. The hurt is blunt and physical.
The mood is raw and quietly bitter. Nothing is dressed up: no metaphors, no dramatic scenery, just two short moments set side by side. The lowercase style and the lack of polish make it feel like a private thought written down in the middle of the pain. The kiss gave false hope, and the speaker is left wounded by how gentle the ending seemed at first.