Character Analysis
Nathan is Will's primary medical caretaker, which means that he spends the most one-on-one time with him out of anyone besides Lou. He also seems to be one of the few people Will treats with a sense of respect.
Why do you think that is? Is it because Will admires his medical expertise? Is it because he's thankful for all of Nathan's help? Maybe. In our eyes, however, Will is so affectionate toward Nathan because Nathan's the only person who doesn't condescend to him and try to make his decisions for him.
We can see this most prominently in Nathan's conversation with Lou about Will's plans for assisted suicide. Take a look:
I felt furious with Nathan, furious with them all. [...] "Why did you even want to come on this trip? Just a nice holiday, was it?"
"No. I want him to live."
"But–"
"But I want him to live if he wants to live. If he doesn't, then by forcing him to carry on [...] we become just another s***ty bunch of people taking away his choices." (22.120-122)
As a friend, Nathan struggles to make peace with Will's decision, but he sees the importance of letting Will choose his own path. That's all the guy ever wanted.
Lest we leave here too bummed out, however, we should remember that Nathan finds some love of his own when he hooks up with a fellow New Zealander named Karen in Mauritius. Who knows if these two will go the distance, but no one deserves love more than this always-kind medical professional.