Although Carl Allen(Jim Carrey) was saying no to everything. Why did Carl Allen decided to go show of Yes Man? What instance made him to go to that show?
Though it's not clearly shown in the movie, I think Carl introspected a little bit, after Peter's intervention.
Carl meets an ex-colleague, Alex, who tells him about the "Yes" seminar which changed his life, and urges Carl to go to this seminar once, and hands him a pamphlet for the same. Carl is not convinced at this point, but keeps the pamphlet.
Later, when Peter comes to Carl's apartment for reprimanding him, for not turning up for his engagement party, Peter reminds Carl, that he has not dated anyone since his last breakup, he doesn't like "doing things", meeting people. Peter tell Carl, that he is still a bank teller, when he should have been at some senior position. This leads Carl thinking for a moment, and then he asks Peter to leave.
IMO, at this point, Carl thinks about all the things Peter said, and remembers what Alex told about "changing his life", and he decides that he'll at least go visit the seminar once, to see what's going on.
It sees from the movie that at first Carl was like other people, natural. But his break up with his girlfriend left him wounded. He decided to somehow punish himself by locking himself up from everybody. Thus he was becoming antisocial day-by-day. It was not like he was doing this unknowingly. Rather this part of him was being made by himself intentionally.
He was so busy keeping himself from others that he even forgot to attend Pete's engagement party. So when Pete visited him and rebuked him on his deliberated ignorance to others, it shook his mind. Later he also had a nightmare in this depicting nobody would regret even his death if he continues to be such a jerk.
So he thought to give the show a shot because his friend Nick earlier that day described him how the show changed his life and filled it with excitement.
So it was not like he was ready to accept its philosophy, but the visit was on a check-only basis merely.