Four short lines, and almost every phrase is a pet name. The speaker addresses someone beloved, though the poem never says who, and calls them "my private bone, my chance heart, my Walden in a wardrobe, my rose." Each name pulls in a different direction. "Bone" is intimate and bodily, something deep inside you. "Chance heart" suggests love that arrived by luck. "Rose" is the old, familiar symbol of romance, which makes the stranger names stand out more.
The most surprising image is "Walden in a wardrobe." Walden is the famous pond associated with solitude and simple living in nature, and here it is squeezed into a cupboard. That suggests a small, enclosed space that somehow holds a whole wild, peaceful world. The "one green bed" at the end carries the same feeling, blending bedroom and woodland.
The mood is tender, quiet, and secretive. The speaker seems to feel that being together through the night is a kind of retreat from the outside world, a private place of calm and growth.