At the risk of starting another debate, I think that Angel just happened to be in the right alley at the right time when Faith finally came to the full realization of what she had truly become, and the determination that she deserved to be punished. Granted, after that point, he gave her shelter and support, but that was only after the fact. Faith had to make the decision and the commitment. No one else could do it for her.
Angel being able to sympathize with her feelings because he, too, had gone through that same phase is great after the dust settles, so to speak, but it isn't the same thing as really showing Faith what she had done and what she had to do to try and make some sort of atonement. If she hadn't been at that point already, one or both of them might never have left that alley alive.