The speaker mourns someone who has died or left, and blames themselves for missing the signs. The poem's repeated line, "I never realised," works like a list of regrets: the smirk, the dimple, the eyes that changed colour when the person was sad. Slowly the details turn darker. The speaker admits to causing pain, and mentions scars on the wrist, which suggests the other person was self-harming and needed help. The poem doesn't say exactly what happened, but "you're gone" implies a loss the speaker can't accept.
The mood is raw, guilty and heartbroken. The short, broken lines feel like someone struggling to get the words out. In the final lines the poem shifts. The speaker says the other person may not have loved them in the same way, but also never noticed that the speaker was hurting and needed saving too. It's grief mixed with quiet hurt, and a plea to be seen.