 Why does Matilda break off her engagement with Herman?
This, in itself, was not unusual—but Matilda was so completely wrapped up in the romantic fallacy of her day that she sought a prince charming, a faithful Don Juan, a man who had been everywhere and tasted of every worldly pleasure and who now wanted to sit on a porch and talk about it all to Matilda. 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!). Someone in Nebraska wanted to trade match books; someone in New York needed a midwestern pen pal, but it was a woman; an elderly man interested in ornithology wanted a young chick correspondent interested in the same subject; a young, personable man wanted an editorial position because he thought he had something to offer the editorial world; and— Matilda read the next one twice. Cedar Falls was a small town not fifty miles from her home, and she'd get there a hop, skip, and jump ahead of her competitors, simply by arriving in person instead of writing a letter. 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....
 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.