In The Bronze Bow, why does Daniel think that he could never marry Thacia?
Daniel's feelings for Thacia have been secondary to his mission, to get revenge against the Romans for killing his father and uncle. Daniel has not allowed himself to act on his feelings, even when he feels that Thacia shares them. Despite this, and despite his knowledge that she would reciprocate if he asked, he is overcome with emotion when Joel tells him that Thacia's father wants to marry her off to someone else.
"She must choose," he burst out now, too harshly. "Someone of her own kind. Your father is right. And you will have to choose too, before long."
"And you?" Joel asked quietly.
"I have no choice. How can a man who is sworn to vengeance and death take a wife?"
(Speare, The Bronze Bow, Google Books)
Daniel believes that he is destined to die seeking revenge; it has empowered him and motivated him for so long that he is not entirely in control of his feelings. He thinks that seeking revenge is his only mission, and so he would be unsuitable for Thacia; he would be unable to make her happy because of his single-minded drive. Fortunately, events allow Daniel to see past his vengeance and grow more emotionally stable, and he allows himself to feel hope for his own future beyond revenge.