As a med student, you meet a lot of new people.
For every rotation, you are obliged to meet at least a hundred new people. There's the attendings, the fellows, the residents, the interns, and the patients. I always found it disconcerting, because it meant I kept running into people who looked very familiar and saying hi, but in actuality having no idea where I knew them from.
For example, on my way to the library one day, I ran into this guy who I was certain I knew, especially after he gave me a big hi. I was searching my brain... he definitely wasn't an intern or a resident... was he some kind of PT? A nurse? Then it hit me: he was the renal fellow, Joe. I felt relieved for remembering.
Then again, as I was leaving, I saw this girl who I was certain I knew. She didn't say hi to me, but she looked so familiar, I was certain she was someone I had met. What's more, I got this distinct feeling that I hadn't liked her when I knew her.
Then it hit me: she was my roommate from one year previously. Damn, what a thing to forgot.
And it was true, I didn't like her.