Actually it makes perfect sense, if you look at it as him faking intimacy by getting it physically without having to commit too much emotionally, but even Jack himself isn't really aware of it quite that explicitly.
A good portion of the tension goes out of him at Spike's response, because here, at last, is a specific problem he can actually attack.
Even if he doesn't really know the answer. "I don't know," he admits, spreading his hands. "I guess not, if he feels I don't. It's hard to know, without a specific example, you know?" He pauses, trying to be really honest, because this is important, and after a moment he admits, rather weakly, "Perhaps I do tend to assume things. Like that the fact he doesn't talk about things means he doesn't want to."
A reasonable enough assumption, he still feels, but apparently not the right one.