The Polish driver is making an incredible return to the sport; after an eight-year absence, with Kubica signing for Williams to drive alongside George Russell for the 2019 F1 season.
"I knew what I would have liked to do, but sometimes things are a bit more complicated than they look from the outside," Kubica explained to Motorsport.com.
"I had some opportunity which I think was very interesting and it was not easy to say no. In the end, I am a racing driver - even though I haven't been racing for quite a long time - and this racing mentality has won out.
"I'm happy to have made this decision because it is a big challenge."
Kubica is adamant that he is well prepared for his return to F1 following a horrific rally-crash back in 2011.
"It's not easy to become an F1 driver; it's not easy to convince people you can do it and establish yourself in an F1 paddock," Kubica continued. "You have to have a lot of drive to do it because I went through this 12 years ago, and in the end, I said I think I have the energy to do it again."