Nan, in Janie Steps In, is fairly shy, emotionally young, and is in a fairly emotionally vulnerable stage, losing the two people who had been the sum of her emotional life up to then. I could see her grabbing onto someone because she's lonely and scared.
Not to mention that Janie and Julian have known each other for about 4 years when they get engaged, and David and Nan have only met on three occasions when Janie has her little talk.
"How many other lads of his age do you know?" Nan gave her a startled look. "Why - not many, of course. Blakeney Place is rather secluded, and anyway I had my lessons and so on and I wasn't thinking about anything of that kind then." "Exactly! You were still a schoolgirl and David was a young man in the Navy. Oh no, Nan! You must have your chance to meet other men and measure David by them - or them by David" she added mentally, though she said nothing about it to Nan. "You haven't had enough experience to judge for yourself. You can't go and get engaged to the first nice boy that asks you."
I can't fault that last line, though I'll bet in RL it's what happened much of the time!
What I always wonder is how old David is at this stage - he's eleven in Seven Scamps
(1927), but I doubt he's meant to be more than in his early or mid twenties in Janie Steps In
. Not thirty-seven, anyway! But then this book certainly appears to be set in the 1930s.
Footnote: Janie's Victorian-style wedding dress must have been a complete visual shock to everyone, given that she presumably married Julian in the late twenties/ early thirties...