The speaker describes a single reaction, and it takes him almost the whole poem to get there. A group of caterpillars clusters under a leaf. When a wasp comes near, they arch their soft bodies and go rigid, bunching together so they look like thorns. They hope the wasp will pass them by. Only in the last few lines do we learn the point: this is what happens to him whenever she smiles.
The comparison is funny, but it also says something true. A smile is usually a pleasant thing, yet the speaker reacts as if it were a threat. He freezes, feels exposed, and tries to look tougher than he is. The "sudden thorns" suggest a clumsy bit of self-protection, since caterpillars are soft and defenseless underneath. The poem's title and subtitle frame it as something said to a friend, so the tone feels like a confession, half-embarrassed and half-amused.
The mood is gentle, close-up, and a little comic. The slow, precise nature detail makes the speaker's helplessness feel real, and the delayed ending lets the joke land softly.