You might want to revisit Chapter Eight of this great book to see the way in which the Chief Elder describes this position to both Jonas and to his community to understand why it is that Jonas has such mixed feelings. On the one hand, it is clear that he has been singled out to occupy a position that brings with it immense honour and respect, and it is a position that requires characteristics that clearly speak of the good character of Jonas. To be selected for such a role over the rest of his year group is thus an immense honour.
However, at the same time, it is clear from what the Chief Elder says that this role is so important because of the way that it involves suffering, that Jonas is very wary and unsure of this role and what it would entail. Let us recall what the Chief Elder says to him when she talks about the pain that such a role will involve:
[You will be faced] with a pain of a magnitude that none of us here can comprehend because it is beyond our experience. The Reciever himself was not able to describe it, only to remind us that you would be faced with it, that you would need immense courage. We cannot prepare you for that.
With such words acting as a caveat about the new position of Jonas, who could blame him from having mixed feelings about his role? On the one hand, he has been selected for a position of honour in his community, however this position will involve great suffering and will also separate him from his friends and family.