He had actually had time to have a long think about why he lost and how he'll have to behave while he's here, the times he was conscious long enough. It's a uniquely awful situation - Sentinel can't quite see how he'd get back home and at the same time, fully realises that there's no way he can rely on his old mindset and expect things to go back to the way they were. Briefly, he mourns the loss of being able to intimidate and murder people. While it's not his only skill, it's certainly the one he enjoys the most.

If he's being honest, he doesn't quite know what to do. Here Optimus was, offering a hand like it was his duty. Like it was his place. Disgusting, really. Sentinel is momentarily filled with enough robot misanthropy he's shocked something didn't spontaneously combust.

He quirks an optical ridge; "Consider the contract sealed." I'm your problem now. If Optimus is going to be suspicious at how easily Sentinel agreed to it, he has good reason.