For Will, it started when they first met after the war.
Will just wanted Nico to be safe, and Nico wouldn't listen to reason. So Will just did what he hated doing and used an argument of authority. His mom would yell his ears off if she knew, but he hadn't found an alternative.
"No Underworld magic. Doctor's orders," he said. And Nico complied.
And he kept complying, every time Will said those two words. It was an innocent in-joke, nothing major or harmful. Will never asked anything Nico didn't want to do—as was evident when he told Nico to go out with him, 'Doctor's orders', and Nico said yes while barely containing his laugh.
For Nico, it started a little bit later. They'd finally started dating, and Nico had been tired of being alone for every meal. So he'd come up with a plan with Will—pretend Nico's loneliness made him lose control of his powers. Will gave him a doctor's note to confirm it, and Chiron…didn't really bother arguing.
After that, it sort of became a thing at Camp Half-Blood. Nico would want something, mention his doctor's note, and get his way.
When Nero's Colossus attacked the camp, and Nico got to make Lord Apollo himself change his plans because of his doctor's note, he realized how much power a little stubbornness could really have.
It was like an unstoppable force and an immovable object. They were bound to meet eventually, and something had to give.
Neither Will nor Nico really remembered what they were fighting over. Just that it escalated, a heated argument leading to more arguing and escalating to a whole week of alternately sniping at each other, ignoring each other, or yelling at each other. All they did know was that suddenly, Nico wanted to leave camp, destination nowhere—which likely meant trouble and/or the Underworld—and Will wouldn't let him.
"You can't leave before we've fixed things between us! Doctor's orders!"
Nico briefly froze, then swiveled on his heels and cast Will his darkest glare. "Oh, really? Well, I have a doctor's note, and it says I need to get out of here right now!"
Two uncontainable forces, but one had to win out. Nico had to stay or leave. The couple was at its point of no return. They stood still, staring at each other for a while, waiting to find out which it would.
"Well? Are you going, then?" Will asked, defiant—daring Nico to go.
"Or you could just calm down for two minutes and talk like mature people who have been dating for a while." Will and Nico froze at the same time, turning to the newcomer's voice—Piper McLean, in full daughter of Aphrodite mode, and looking fairly miffed. "Seriously, guys? Everyone at camp called me back from New Rome because you were freaking them out so much. What's going on?"
They glanced at each other, silent and confused, then back at Piper. Finally, Nico spoke. "I just—need to get away."
Piper turned to Will, who shifted uncomfortable. "And I don't want him to leave."
She glanced back at Nico. "You want to get away. Away from Will?"
Nico paused. "No. Just—away from here."
Piper looked at Will again. "You don't want him to leave here?"
"I don't want him to leave me."
She finally encompassed the two of them in a single, deadpan glare. "Sounds like your solution is simple. Just leave together. Go on a quest, or a vacation, I don't care." The boys stared at her, stunned—and feeling pretty stupid. "And please, in the name of all the gods on Olympus, in the Underworld, and wherever else there are Gods…enough with the fucking catchphrases. It stopped being funny about five minutes before you first used them. Try talking like actual human beings for a change."