Or luck, more like it. The Whirlwind was still mostly intact by then, even if Angelus lost his sense of fun along with his soul. Encountering someone like Jack would've been fun with possibly disastrous results. As for choosing to fight vampires... no Slayer chooses that life. It choses them.
He rolls up to stand as Jack pushes him off, bouncing on the balls of his feet — yeah, this is fun. It's been too long since they had a good old-fashioned rumble, just the two of them, and this is the best place to pick it up again.
"You're not pretty enough to play the Slayer." He grins. Jack is no China doll. "But alright... if you really want to feel what it's like, who am I to say no?"