Jonas liked Fiona as a friend, but he came to have stronger feelings for her.
In Jonas’s community, no one has strong emotions. Jonas is friends with Fiona because she is his age and she is nice to him. One day he volunteers with her at the House of Old. That night he dreamed about Fiona, and his parents explained it was his first Stirring.
Finally she sat down beside him at the table. "Jonas," she said with a smile, "the feeling you described as the wanting? It was your first Stirrings. Father and I have been expecting it to happen to you. It happens to everyone." (Ch. 5)
Stirrings are sexual feelings that come with the onset of puberty. The community wants to limit its citizens’ experiences and feelings. This is one of the reasons for Stirrings pills. The other reason is that they do not want any unexpected babies if people act on these feelings.
Jonas’s feelings for Fiona are more complex. When he learns that release means death by lethal injection, he also learns that Fiona has been trained in release.
"And what about Fiona? She loves the Old! She's in training to care for them. Does she know yet? What will she do when she finds out? How will she feel?" Jonas brushed wetness from his face with the back of one hand. (Ch. 20)
The Giver explains to Jonas that Fiona has not been trained to feel. There is no such thing as feelings for Fiona. She is just like everyone else in the community in that she thinks nothing of releasing the elderly. She has no concept of death.
For Jonas, learning that Fiona participates in release is just another betrayal. He is horrified by his community's actions, but knowing that Fiona takes part in the procedure is even worse. He always considered her a good person and a friend. He realizes that both his father and Fiona are not who he thought they were.