We all like to think that we're special, but in reality we're much more alike than we like to admit.
We all have to breath and eat and sleep. The only thing that sets us apart is how self centered we can be.
Spike did it for Buffy, so I guess his motivation would be lust.
Willow started out Ok, but it became a pursuit for power.
Giles and Buffy did it because of destiny. With out that, I know Buffy wouldn't have did it.
I did it because some monks decided to make the mystical into flesh.
Mr. Wood did it for revenge.
Faith did it for destiny also, but now I think there's a little desire for redemption in there too.
Anya did it for lust, and greed.
Xander started out doing it for lust. Like Spike he did it for Buffy but along the way he kept doing it for us all.
So out of many, one did it for less selfish reasons.