After going through an absolutely massive life change, Jess left her home and moved into a loft with three guys. That was the premise of New Girl. But it was that first episode that set the stage as well as the destiny of some of its key characters. Most importantly, Nick made a big decision in that pilot that ended up defining the course of the entire series, not to mention his relationship with Jess.
Discovering that a partner has been cheating would be devastating for anyone to deal with. When Jessica Day caught her boyfriend cheating on her, she lost a lot more than a relationship; she also lost her home as she moved out and left many of her possessions behind. But it turned out that she gained so much more when she answered a Craigslist ad for three guys looking for a roommate.
This was how Jess met Nick, Coach and Schmidt. Coach was suddenly gone after that first episode and was replaced by Winston, but still, this group of guys became some of Jess' closest friends, in addition to being a key part of her support group. Nick and Jess became extremely close quite quickly, and it was evident that a romantic relationship was building between the two of them thanks to Nick choosing Jess over his ex.
The fact that Jess and Nick were going to end up together was clearly laid out for fans in Season 1, Episode 1. At that point, Nick was still pining away for his ex-girlfriend, Caroline. Their off-and-on relationship was a big part of the early seasons of New Girl. In that first episode, Nick, Schmidt and Coach needed Caroline's assistance to get into the party of the year. She agreed to help them, giving Nick a chance to reconnect with her.
At the same time, Jess was about to be stood up on a date. When Nick found out, he abandoned the party and Caroline to help Jess, with Coach and Schmidt in tow. This was the first stage of what happens between Nick and Jess over the next seven seasons. He consistently put Jess first in every possible situation, even after they broke up in Season 3, Episode 20.
In Season 2, Episode 14, Nick became worried that Jess was teaching creative writing to someone who wanted to kill her, so he broke into the guy's shed. Jess loved birthdays but avoided her own so Nick threw her the perfect party in Season 3, Episode 13. As strange as this sounds, he even opened a bank account because she wanted him to in Season 3, Episode 5. Nick was a notorious conspiracy theorist and had long avoided an account, so this was a big deal for him.
These examples are just a drop in the bucket when it comes to all of the ways Nick always put Jess ahead of everyone and everything else in his life. And it all started with one decision in that first episode. Even Caroline knew what was up when she returned in Season 3, Episode 15. It was in that same episode that Nick revealed he had fallen for Jess in the pilot the moment he saw her and never really looked back.