In just three short lines, Donna H describes Alzheimer's disease as a thief. The idea is simple but powerful: memories are things we own, and here they are taken away without permission. The thief is "unknown," which suggests that nobody can see it coming, name it, or catch it. That leaves the speaker, and anyone affected by the illness, with nothing to blame or fight against.
The speaker seems to feel anger mixed with sadness and helplessness. The word "cruel" shows how unfair the loss feels, and the last line, "for no good reason," makes it sharper. There is no explanation and no purpose behind it, only loss. The poem's brevity adds to this feeling. It is as if the poem itself has been stripped down, with little left, much like a mind losing its memories. The mood is quiet, bleak, and heavy, and it ends without comfort or resolution.