Sonic tells his past self to enjoy his future and that it's going to be great...
Since everyone seems to be stumped by the presence of Sonic 2006's Crisis City even after it was supposedly "erased" from Sonic's timeline, I thought I'd put the whole explanation about it in the description. :D
Technically, the events of Sonic '06 WILL happen because they HAD to happen for them not to happen. Sonic HAD to go through those events so that he and Elise could reset time so that the events of Sonic '06 would never happen. If Sonic '06 never happened, then Crisis City wouldn't have been a playable area in Generations, the Time Eater took Crisis City from the "erased" timeline of Sonic '06 so it happened but it didn't happen. STRANGE ISN'T IT?!