The speaker insists on one point: they didn't stop loving this person. The love didn't fade or run out. What happened is that the person they loved seems to have disappeared. The first half of the poem lists what was loved, including dreams, flaws, a beaming smile, and dark brown eyes the speaker says they could never forget. Then the poem turns on "But," and the details start to unravel. The eyes aren't as deep a brown, the smile is gone, and "You are not him anymore."
The strangest and most touching moment is the line about atoms, where the speaker wonders whether the person even consists of the same matter as before. It's an exaggeration, but it shows how total the change feels. The tone is quiet and sad, with a little stubborn pride in the repeated denial of "falling out of love." The ending is calm but unresolved: "I'm just learning to live with that." It sounds like grief for someone who is still alive.