The sad part for me (because I totally admit to being a Spike-first person) is that I cannot think of a way that I would even want him to react. There's no reaction he could have that's the right one. If he's angry with Angel, he's 'jealous'. If he's angry with Buffy, well, he never told her (himself) that he wasn't dead so he can't be angry with her. So both those reactions are out. If he's okay with what's gone on, well, sheesh. But he can hardly be self-righteous about any of it. And yet we've basically been left with the feeling that neither Buffy nor Angel gave him even a fleeting thought (nor have we ever seen any evidence whatsoever that Buffy mourned in any way).

So basically, there's no good reaction for Spike to have that doesn't paint him jealous, envious, self-righteous, unjustified, or pathetic. He just isn't left with a lot of options.