Dr. Ivo Robotnik: I discovered the source of ultimate power.
Agent Stone: That sounds big.
Dr. Ivo Robotnik: It's been on my vision board for years.
Tom: Sonic, I love that you want to help make a difference. But you're being reckless. Pretending to be Batman?
Sonic: Blue Justice! Trademark pending.
Tom: You're still just a kid. Trust me, there will come a moment when your power will be needed. But you don't choose that moment, that moment chooses you.
Sonic: Today's forecast calls for a 100% chance of adventure.
Dr. Ivo Robotnik: Papa's got a brand new 'stache.
Answer: Because Robotnik was betraying him. Dr. Robotnik promised to help Knuckles get the Master Emerald, so they could use it to defeat Sonic. But Robotnik was using him all along because he wanted the Master Emerald for himself... he never actually cared about Knuckles and never intended to help him get the Emerald. Thus, Knuckles is shocked and says that Robotnik taking the Emerald for himself was not part of their deal.