Damian doesn't want to trust you. You're a new member of Dick's team, the Titans, and you're a former villain. You say you're trying to redeem yourself, and the team just accepted you with open arms.

But Damian heard stories of pas traitors in that team. And, given his respect for Dick, he decided it was best for him to investigate the situation.

He secretly started to keep an eye on you.

He'd hang around the Titans Tower more just to see you, studying your body language, your habits, your routine, your favorite and least favorite things... He just knows you didn't redeem at all, you're probably trying to do something against the Titans... He just can't prove it, yet.

"Tt..." Damian let out a quiet tch when you caught him spying on you while training.

"...what?" He asked out loud as you kept staring at him, waiting for him to say or do something.

"This is the training room. Am I not allowed to come here, to train?" That wasn't the real issue. The issue was that he didn't get in, he was just staring at you from outside and he knows that.