1600s

Bella had not meant for things to go so far with him. She wasn't looking for a mate, especially not such a strange one. She'd be lying though if she said she didn't enjoy it. Carlisle was a quick study when it came to most things and sex was no exception.

It only took a few days for him to become proficient in the act of sex, two weeks for him to be up to her standards in fighting. Bella was pleased with his control of his blood lust. If things kept going at this pace she'd be out of things to teach him all too soon.

All the sex was slowing down her lessons, but making things much more interesting.

A month after they first started enjoying eachother's bodies Bella called out to him again as she walked toward her house. He walked out and froze when he saw she was carrying a drained human.

"So I was thinking," Bella said as she dropped the corpse in front of him, "if you really care about the humans as much as you claim to you should become a doctor. With your enhanced senses you'd be able to better identify and understand their ailments. Your years of practicing would also help. And if you're really good at your job you might be capable of offsetting at least half of the deaths I cause." Bella laughed loudly before digging into her pocket and pulling out a foldable knife. She tossed it to Carlisle who was still so shocked that it just hit his chest and fell to the ground.

"I don't think I can-"

"Don't be ridiculous. Of course you can cut him open, he's dead! How else are you going to learn how they work if you don't know what's going on inside them? Anyways, it will be good practice for your self-control considering I'm sure there's at least a little bit of blood left." Bella sounded like she had all the confidence in the world in him, and Carlisle almost believed he could do it. With shaky hands he picked up the knife.

Kneeling next to the corpse he felt a little woozy. He'd never seen a cadaver so close before. He'd spent all his vampiric life avoiding humans. Bella ripped the dead man's shirt open and pointed to where he was to begin. Carlisle was sickened by how easily he could glide the knife through the man's flesh. Bella showed him how to open up the ribcage and named off every organ and its functions as Carlisle found them.

It was an experience he would never forget.

I always wondered how Carlisle decided to be a doctor... so I wrote out my own answer!