Maybe it feels safer to Jack, but it makes Sinclair just a tad nervous to not be able to hear anything at all over all the machinery in here. It's not so loud that he couldn't hear Jack speaking right next to him, but as far as splicers in the distance go...

On the other hand, this is a place he remembers. It's hard to forget a sight like this, something like witnessing your own open heart surgery. All the tubes and pulses you don't really understand, you just know that this is what keeps you going. He came down here before Persephone started undergoing construction, a couple mechanics showed him around at his request. He needed to know how much he would be relying on the rest of Rapture for his own needs. Obviously the less, the better.

Jack has a hand to his temple, pushing down like he's trying to snuff out his own thoughts. The ones that bounce around in your skull, kicking and screaming until you figure out how to silence them or you take too long and they get bored. The latter is considerably less pleasant.

And really, ignoring what's going on here any longer is not going to solve anything. Quite possibly, it can only make things worse. And Jack strikes him as the type who might be too proud to just say something.

"You ought to stop and find yourself a sea slug, son. You're lookin' a little pale."