Answer:
The Giver tells Jonas this because he is thinking that maybe it would be a good idea for the whole community to have memories again, and that he could help them cope with them.
Explanation:
When The Giver tells Jonas the story of Rosemary, he tells him that when she asked for her release, all the memories she had absorbed for 5 weeks returned directly to the community all at once. This caused great chaos, and with the pain that The Giver felt and the anger, he did not want to help them.
Now, he is deep in his thoughts because if something happened to Jonas, the memories of a whole year would pass directly to the minds of each person in the community. This could be a big problem, but what if The Giver helped them this time?
It's a considerable option, and that's why he tells Jonas that they can talk more about it sometime and that he needs to think about it some more.