As Spike curled around her, purring, Willow thought that vampires where basically overgrown cats. They purred when content, as Spike was doing now, and at other times. Petting Spike was guaranteed to get a purr and sometimes a nuzzle. She enjoyed the cat baths, err tongue lickings she received when injured on patrol.
Vampires hunt like cats. Some hunt in packs, and others hunt solo. Both play with their food before killing it and kill prey in pretty much the same way. Although a cat's killing bite was quicker than a vamp sucking your blood out. Both have enhanced sight and smell to detect prey. Both have extra senses beyond the normal five: cats have whiskers and vampires can just sense things. Vampires were limber as cats and able to fit into just about any space, especially when hiding.
Glowing yellow eyes were another similarity and similar in shape.
She had heard Spike growl a few times when Willow was threatened on patrol,
and
hissing during a fight. Fangs. Both had extremely sharp fangs
although being bit by Spike was more pleasurable then being bit by that
stray cat. The sense of taste was just as well developed as evidenced
by Spike's complaining about how cow and pig blood tasted horrible.
Stealth was an ability both shared. She hated when Spike snuck up on her and scared her. She knew that that the stealth came with the hunting ability, but it still annoyed her. Hunters had to be silent or they would chase away their prey.
They slept all day and wrecked havoc at night. She had been woken
up quite a few times at night by a bored vampire. Basically she was
living with an overgrown, blonde cat that drove her wild and stole her
heart.