Why did he think he could do this sport
because Punk legit thought he was a tough guy and bought into his own bullshyt. just because someone talks a tough game or is an a$$hole to everyone doesn't mean they can fight. you would think he would have learned that after Teddy Hart beat his ass backstage years ago, but again he bought into his own hype.
the truth is that he would easily get his ass handed to him by 90% of the male roster in WWE and likely 10-15% of the female roster as well. just look at his hands compared to E and Cena here... that little, angry bytch isn't going to hurt ANYONE