Great question! What is key to think about is that in this chapter the Giver gives Jonas his favourite memory, which is of Christmas and a family together. The overwhelming impression Jonas has of this memory is warmth and love - two emotions that are absent from the life of the community. This encourages Jonas to begin to think about the differences between life "back then" in our time, and his life now in his community, and he begins to see that there are some things that he feels should be changed.
This change in Jonas is demonstrated most clearly when he lies to his parents for the first time ever, after he asks them if they "love" him. At the end of the Chapter, Jonas talks to Gabriel, and makes clear his ideas for change:
"Things could change, Gabe," Jonas went on. "Things could be different. I don't know how, but there must be some way for things to be different. There could be colours.
"And grandparents," he added, staring through the dimness towards the ceiling of his sleeping room. "And everybody would have the memories."
Thus Jonas has changed greatly since he was given his particular assignment - he has experienced memories which have helped him to identify some of the shortcomings of their way of life now. He has started to desire change.