At first she just wanted "something sweet" for the night, but she learned that this guy was different. I think knowing only that he was the new Lord Rahl or that he was a good guy wouldn't have been enough to make her join the team in Marked, she needed both sides of the story. Then she started trusting him more and more, Broken was a big milestone. We got to see some bonding moments but not enough imho. Though I don't mind that she had more scenes with Kahlan. I guess they wanted to explore this friendship more in season 3.