In The Bronze Bow, why does Daniel feel that he is unfit for marriage?
Daniel has lived his entire life in pursuit of one goal: revenge against the Romans who killed his father and uncle. Because of this, he doesn't see himself as a worthy prospect for marriage, not for himself or for any one else. Daniel believes that his goals in life make him undesirable, and that he would end up making a wife unhappy. In this sense, his belief is altruistic; he wants to suffer alone, not to drag anyone else into it.
"I only know one way to fight," he said. "I don't have words like Joel's. I have only my two hands."
Her voice broke. "Will there never be an end to it -- the hate and the killing?"
"Thacia!" he burst out. "Don't torment me! I have to see it through alone. There's no room for anyone else."
(Speare, The Bronze Bow, Google Books)
However, he has not yet learned to open his heart to love, and so his view is still immature. An adult knows that the way to shoulder a burden is to share it with people who truly care, both about themselves and about others. Daniel cares, but selfishly; he wants his mission to be about himself. When he learns to open his heart to love, he realizes that he needs to care about others as well; his goal is not the only important thing in the world. Despite his thoughts of personal sacrifice, he is only wasting his life; when he shares it with others, Daniel is able to make his life count for something other than revenge.