How did Jonas decide he felt? What was causing this feeling?
I don't know at what point along the story line you are referring to, but at the very beginning the book states that Jonas has been feeling 'apprehensive.' (It took him a while to find the right word.) This is because of the approaching Ceremony of Twelve, a coming of age ritual in which he will be given his Assignment. As everything else in the Community, this is not a question of choice but appointment; he will be delegated his job to do for the rest of his life. Jonas is understandably anxious in that this moment is an important turning point in his life over which he has absolutely no control.
The above answer is a pertinent one but it comes later on, after Jonas has been appointed to be the next Giver in line and is in training as the Receiver. His feelings of anxiety (over the unknown) give way to anguish, then horror, as he is imparted the collective memory of a very dystopian society. It is his "wake up call" to change the course of events by giving the Community back its past, and consequently, its ability to reason and choose.
After a particularly intense session where the Giver shares the horrors of war with Jonas, shown through the visions of dying men and horses strewn across a battlefield, and Jonas himself experiencing an agonizing injury, they move on to a scene of Christmastime. This is the Giver's favorite memory. Jonas is struck by the idea of parents, grandparents, and children spending time together, opening presents, being part of each other's lives. Jonas realizes that in his society, one doesn't even know what happens to one's parents, or when they are released. He likes the feeling of this memory, and the Giver tells him that this feeling is love. When Jonas asks his family about the concept of love, they ask him to use more precise language, and explain that they are proud of him, but that they don't love him.