The narrator desperately wants to make Lettie feel like he’s still a worthy friend who can do as he’s told. He wants to be able to accompany her next time, so he needs to act as mature as he can. Lettie, however, behaves as though this isn’t really a big deal. This speaks to Lettie’s deeper understanding of what’s going on. To her, Ursula isn’t as frightening as she is for the narrator; she’s just a flea, and Lettie has dealt with fleas before. Because of her perspective, none of this is quite as scary as it is to the narrator.