He can't even find words to respond to that, so he just lets out a furious yell, going at Spike like a crazed man, just letting it all out, all that anger and exasperation that he needs to let go of before he can even start to think about this rationally. He's hurt and he's despairing and he's drunk, and really this is what he needs.
Though Spike gets in his punch eventually and Jack goes rearing back, as far as he can with Spike's other hand still hanging onto his coat, groaning as pain blossoms in his jaw.
But he's a lot calmer as he sits up, blinking down at Spike as if he's not quite sure what just happened there. "Uh..."