This emotion is love.  When Jonas experiences the memory of grandparents, he realizes that there is an emotion in the room that he has not experienced before.  He is perplexed.  Jonas has never experienced love, because it does not exist in the community.  This occurs in chapter 16, page 123, when The Giver shares his favorite memory.

When The Giver asks Jonas what he is feeling, Jonas is uncertain:

“Warmth,” Jonas replied, “and happiness.  And—let me think.  Family.  That it was a celebration of some sort, a holiday.  And something else—I can’t quite get the word for it.”

And on page 125:

Jonas hesitated.  “I certainly liked the memory, though.  I can see why it’s your favorite.  I couldn’t quite get the word for the whole feeling of it, the feeling that was so strong in that room.”

“Love,” The Giver told him.

Jonas repeated it.  “Love.”  It was a word and concept new to him.

Jonas considers the impracticality of grandparents, but still likes the idea of love.  On page 126:

“I liked the feeling of love,” he confessed.

Jonas states that the memory shows a dangerous way to live. He says he is talking about the candle and the fire, but he is really talking about love.  Love is dangerous because it leads to attachment.  Attachment makes control difficult.  There is no attachment in the community, and control is considered most important.