Yea but "chuck" calls him "teddy" after "teddy" calls him "chuck".  So he was going along with it because DiCaprio is crazy, and a psychiatrist usually doesn't want their patients going into a raging fit.
And did Teddy give himself up at the end?  He said something along the lines of "Is it better to die a hero, or better to live a monster" and then he willingly walks off with the doctor to get the lobotomy.  I think that he gave up because he could not accept the fact that he was crazy and imagined basically everything that happened in the movie.
I thought it was an excellent movie, but the trailer definitely ruined the whole first half.  It made everything predictable until the end.  I also kept getting confused about if he was really crazy or if they were making him think he was.  I want to see it again, such a good movie.