 Why does Matilda break off her engagement with Herman?
The first thing the widow Penshaws did was to take Matilda's left hand in her own and examine the next-to-the-last finger. "But, Matilda, that's your fifth broken engagement in three years. Matilda switched her bed lamp on and dabbed some citronella on each ear lobe and a little droplet on her chin (how she hated insects!). Twenty years from now, this could be Matilda Penshaws—complete with plain gray dress, rimless spectacles, gray hair, suspicious eyes, and a broom-stick figure.... There were five cars parked in the long driveway, and now Matilda's made the sixth.
 He was too “stuffy” for Matilda.
 She’s looking for someone more adventurous.  He was too “stuffy” for Matilda.  She doesn’t want to settle for him, as she’s too fixated on the idea of romance she has.  She’s afraid of commitment, as is hinted by this being another broken engagement for her.