"I didn't want you there if it went south." That, at least, was completely genuine. He and Sam had seen each other in bad places before, but wasn't that more of an excuse not to want him around? Sam had already seen him die too many times. If Dean could avoid adding to that, he would.
All other selfish reasons aside. Obviously.
"I told you I would try it, I just didn't say when. You can talk to Illyana if you want to, she'll probably have some kind of crap to say that I won't. This isn't a lesson, I just didn't need you to have another story about that time Dean Hulked out or that time Dean died." Illyana was more likely to give Sam detail that Dean didn't want to admit to, but Sam would go to her anyway. He might as well endorse it and get a little good will back.