I guess the sentence actually reads:
Il faut nous promettre de ne pas nous réserver autant d’émotions.
The meaning is something like:
You have to promise not to keep so many surprises (thrills?) for us.
or (perhaps?), as suggested by @Lambie
You have to promise us not to harbor so many feelings towards us.
But you must promise not to provide us with so much excitement.
Here is an excerpt from the original book: