There's no mention of Mark having a romantic relationship on Earth. Do you think that makes it easier or harder to endure his isolation? How would the story be different if he was in love with someone back home?

I believe that Mark Watney's lack of a personal love life on Earth made it easier for him to endure the struggles of isolation on Mars. Mark Watney's was used to being alone in a sense, obviously he had friends and loved ones on planet Earth, but he had no significant other to spend large amounts of his time on and with. Watney's "significant other" was essentially the space program and his training, so once he got to Mars, it was basically everything he had trained to do even though things went a little differently than originally planned. Therefore, if Mark Watney had been married or had a girlfriend back home on Earth, he would have struggled quite a lot more since he was missing a lot more than the planet and his friends, but also, his significant other. The story would have had a lot more of emotional breakdowns due to the fact Mark Watney would be extremely depressed due to lack of emotional connection and just missing being around his person 24/7. Now the struggles would have been harder for Mark Watney but at the same time he would have had a lot more motivation to get home to Earth, to see his loved one! I believe if Mark Watney had a loved one back home, he would have had a bit of a tougher time initially dealing with the isolation, but then he would have had a surge of energy and motivation to get himself home. The struggles would be similar for each version of Mark Watney's life though, with someone back home the initial setback of depression would set him back so much that his secondary surge of motivation would have only leveled him back out to how he would have been if he did not have someone special back home.