Wash says nothing when York finishes his sentence. Heightened perception tends to go hand in hand with giving a shit, so he really shouldn't be surprised; either way, he's not about to confirm it. There's a difference between I don't think and I'm not, Wash knows exactly what side of that line he falls on, and nobody else needs to hear about it.

The next bit is logical and requires an answer. "And that's why I'm not." Sometimes, it pays to be able to lie.