Even though Sam was born to be Lucifer's vessel, Lucifer needed to take some precautions. So he had Azazel create the special children, in addition to freeing Lilith. Azazel managed to open a gate to Hell, and Lilith was set free. The next step was to get Dean to Hell, so he could break the first seal. Once the first seal was broken, Lilith began breaking the rest of the 66 seals so Lucifer himself could rise. Sam was the one who broke the final seal, thus setting Lucifer free.