The 25-year-old said although he played almost every game with the Kangaroos, he found he was getting pushed out of the midfield more often than he liked due to the depth of North Melbourne's midfield.
He finished third in North Melbourne's best and fairest and looked to have cemented a spot in the team's best 22 but he was not sure of his future status.
It made the offer of a four-year deal from Collingwood attractive when the season finished.
He said the discussions with North Melbourne over his next contract did drag on but he only made the decision to leave the club in the past three weeks.
"Even though I did have a full season at North I did find that I was getting pushed out a bit and we (North Melbourne) go very deep in the middle, and just the four-year security was a big motivator as well," Greenwood said on Wednesday.
Greenwood admitted leaving a club that had reached the preliminary final after his best year meant he faced more pressure in 2015.
"Them (Collingwood) showing the faith of four years in me puts the pressure back on me and I definitely want to give something back now," he said.
Collingwood coach Nathan Buckley said the Magpies were interested in recruiting Greenwood regardless of whether Dayne Beams left the club or not.
"We're always looking to add to our list and give ourselves the best chance of success," Buckley said.
Travis Varcoe, Levi Greenwood, Nathan Buckley and Jack Crisp. Picture: Getty Images