The 35-year-old said his retirement from representative football in 2018 was a big factor in his decision.
"I was feeling really good after the 2018 season, given it was my first time in a long time away from the representative arena," Smith said on Thursday.
"Finishing with no injuries ... I'm feeling as good as I've ever felt and that was the reason I asked the club for two years and, thankfully, that was an option that worked for them as well.
"If it was a deadset 'no' from the club then I would have been happy to go ahead with the 12 months."
Smith admitted the club was initially surprised by his two-year call but said there was no acrimony.
He said the lengthy negotiations were due to the club working out its salary cap heading into 2020 and logistics with him travelling on an extended holiday overseas with his family.
Set to turn 36 in June, Smith is older than new assistant coach and his former teammate Ryan Hinchcliffe.
Age aside, he's on target to set to some NRL records that will be extremely hard to beat.
He already holds the record for most games at 384 and, with more representative commitments, should take that mark beyond 430.
He is also poised to become the NRL points record holder. He is second (2386 points) only to ex-Canterbury ace Hazem El Masri (2418).