House was still House, and Kelly had spent years of her life devoted to rehabilitating those who were just as mean and nasty and damaged as he was, and some even worse. As such, their trip was not completely lacking in tension.
House was used to feeding off of people's reactions to his abrasive and offensive demeanor, and when the mood struck him her NON-reactions drove him crazy. And so he would push harder and be even more offensive in his determination to get a rise out of her. But she was stubborn and well practiced at keeping her cool, and so the only time he really succeeded was by accident, when he made a remark --- for him it was just honest, but she was incensed at how rude and ignorant it sounded --- about a street person they saw.
First she fell silent, steaming. And finally she pulled the car over and let him have it. And when he found himself laughing out loud, not at what she was saying but at the fact that she had finally blown up, she gave him some more.
He fought back, defending himself, doing what he always did to squash his opponents. Until he made her cry, and didn't care.
House was still House.
There was also the issue of religion, her conservative evangelical upbringing clashing with not only his atheism but his belief that anyone who was not an atheist was by default an idiot. Her unwillingness to debate it with him was annoying.
"Shouldn't you be trying to convince me?"
"Why, when you're not willing to be convinced?"
"But -" And he would go on to tear apart her belief system, to argue why he knew he was right. She listened respectfully, ask questions to clarify his position, and offered little to counter him. Which only made him more annoyed. "Aren't you going to say anything?"
"There are lots of things that I'd love to share with you, because it's a big part of who I am," she told him honestly. "But I know you don't really want to hear about it; you just want to pick a fight."
"So what if I do? If you believe it so strongly, shouldn't you be willing to fight about it?"
She just smiled. "It's because I believe it so strongly that I DON'T feel the need to fight about it. I don't feel threatened by you. If anything, it just makes me feel sad that you hold so strongly to something when it obviously doesn't make you happy."
"Truth isn't about being happy," he countered easily. "It's about being right."
"So you'd rather me think like you and be miserable than keep thinking like me and be content and at peace with the world and with myself. Thanks, House. I'm so glad to know you care."
"You know that's not why-"
"Are you sure? Maybe you'd better think on that some more."
He did. And he didn't bring it up again.
But House was still House, and he had to be right about everything. So whenever they disagreed on the smallest detail or fact, he had to make it a big deal to try to get her to concede. She'd always solved these kind of arguments -- with her husband or the girls at the shelter -- by turning to the Internet, so in the evenings she'd hit Google while he was in the shower and copy the ACTUAL facts into a Word file to recite to him later. He HATED that she did this, but she hated that he always came on so strong.
So there was tension, and with another pair it might have been enough to ruin an otherwise wonderful journey. But House was still House, and Kelly was still Kelly: he was quick to forget and she was quick to forgive.