Luffy and Roger are both pirates who were smiling on the execution platform in Loguetown. Their personalities and character qualities are eerily similar. Later in the anime, we discover that even the Dark King, Rayleigh, supports Luffy and says he reminds him of Gol D. Roger. So it can be concluded that these two great men had met before they were born. They have been following their dreams since they were young kids and they aren't about to stop now. If anything, they seem to be even more determined than before to become King of the Pirates.