McIlroy added he "certainly can't relax" this week at Oakmont, where he missed the cut when the challenging Pittsburgh layout last staged the US Open in 2016.
He also failed to make the weekend in 2017 and 2018, but has since posted six consecutive top-10 finishes and has been runner-up the past two years.
"I made the decision at that back end of 2018 into 2019, I wanted to try to build my game around the toughest tests that we have in the game," he said.
"The US Open went from probably my least favourite major to probably my favourite because of what it asks from you, and I love that challenge."
While McIlroy said he tried to wipe 2016 from his memory, he was handed a stark reminder of Oakmont's severe demands - five-inch rough and undulating greens - when he shot an 11-over 81 during an early practice round last week.
"Last Monday felt impossible - I birdied the last two holes for 81," said the five-time major winner, adding the course feels "softer" this week.
"It felt pretty good, it didn't feel like I played that bad! It's much more benign now.
"They had the pins in dicey positions and the greens were running at 15 and a half. It was nearly impossible."