Wandering through a single line, this poem sets two opposite images side by side and asks how they can belong to the same person. The "highlight of my dreams" suggests someone the speaker longs for, someone who makes sleep or imagination feel bright and special. The "monster under my bed" suggests the opposite: a hidden threat, something childlike in its fear but still unsettling.
The speaker seems caught between attraction and fear, or between love and hurt. The question isn't really asking for an answer. It sounds more like confusion, maybe pain, as if the speaker can't make sense of how someone so wonderful can also cause dread. Both images belong to the same place, sleep and the bedroom, so there's no escape from either one.
The mood is intimate, uneasy and a little wounded. Because the poem is so short, the contrast hits quickly and lingers, leaving the reader with the same unresolved feeling the speaker has.