I think that you are probably asking about the part in Chapter 10 when Jonas first goes to start training for his new "job" as Receiver of Memories. If that's what you are asking about, the best answer I have is that Jonas cannot (at that point) understand the idea that the Giver could carry the memories of all these times past that Jonas doesn't even know about.
In the community where Jonas lives, everything has been the same pretty much forever (as far as anyone but the Receiver knows). So Jonas doesn't think of the fact that there could have been other times and other places when things were different. If everything has always been the same, what would be the point of having these memories -- they would all just be the same as things are now (except for little personal memories, which is what Jonas expects).
So the sameness that Jonas has grown up in makes him think that the way things are here and now are how they have been always and everywhere. Therefore, he can't anticipate what kinds of memories the Giver really has.