Spike is just going to take a moment to light up and smoke and allow Jack some time to work through this existential crisis. Spike would never have gotten this far in life if he had any doubts about who he is when it comes down to it and so he can in some ways understand what's happening here. It's easy to get lost.
"Probably talked about this before, but..." he pauses, takes a drag. "...in my world, humanity gets you killed. As a vampire you can't show that you care, can't be soft, so if you still have all those human emotions you have to put up a front. Get hard, while remembering to turn that off around the people you really care about."