His friendly talk with Teller was a way to gain Teller's trust so he could publicly destroy Teller for destroying him and not being man enough to take it, unlike even the kid who committed suicide who took it, survived it without ing out and filing emotional charges against him.
He didn't want Teller to be his Bird. He wanted to remind Teller he is without him, and Teller blinks momentarily, giving Simmons endless delight as he kicks him not having it before the audience. He is surprised when Teller comes back and likely figures it will add to the tale, when it takes a turn you see the cognitive dissonance on Simmons face as he grudgingly is complicit to increasingly involved in the success.
Another reason why the movie is great, is that in all that happens, the why of it happening is open to individual perception and interpretation.