The 32-year-old forward has clad himself in red and green since 2018 but was granted a release from the final year of his contract with the club to join former coach Wayne Bennett at The Dolphins.
"It was definitely a difficult decision. It was one that I wrestled with over the Christmas break," Nicholls admitted to AAP.
"I had some time off and was around the family and was able to weigh up the pros and cons.
"Souths had said that there probably wasn't the money there that I maybe deserved to be on. And then obviously at the end of the year, the new team was announced and I knew Wayne was going, so it probably went from there."
"One of the things that made the decision easier was I knew I still had 12 months and didn't have to go straight away."
Nicholls was one of Bennett's first signings to the NRL's newest franchise, joined by the likes of Jesse Bromwich in an experienced front line.
The Dolphins have also added Kenny Bromwich and Felise Kaufusi from the Storm as they added plenty of leadership to their forward back.
Though the end is near for Nicholls' time at Souths, he is resolved to postpone his Rabbitohs swansong for another week as the Rabbitohs seek a spot in the 2022 grand final.
"It'd be disappointing to end this weekend," Nicholls continued, "It's something you think about during the week (that it could be the last at the club), but I haven't actually thought about it on game day."
"We've got a pretty tough job ahead of us this weekend. But we also know what the reward is at the end of it.
"There would be no better way to go out than win a grand final at South Sydney."