it must be so hard for buffy to watch spikes reform grow from his love for her (however obsessive and strange it is) when that is exactly what she could not do for angel. she tried, hard, but there was no amount she could love unsouled angel that would turn him back, and no amount she could love souled angel that would make him stay. I think this is what reveals something underneath the "soul" concept in btvs - it's actually about what remains of the underlying humanity, and excavating that from the layers of vampirism. angel was a cruel, womanizing drunk. spike was a hopeless obsessive romantic who liked pain. vampirism is just a guise, an opportunity to lean into the worst manifestations of those impulses.