Because it's the easiest thing you can do to impress people. This isn't high school anymore.
  1. Repeat it.
  2. Compliment it immediately. "Your name is Calendar? Oh, that's cool."
  3. Ask them how to spell it. "Charity, is that C-h-a-r-i-t-y?" You don't really want to do this with a name like "Adam". But you can.
  4. Next time you see them at the party or wherever you met them, say it again. Over do it even. "Beth! My new friend Beth! There you are, next to the denim beanbag chair!" @samboyd
  5. Make the effort. We're intelligent beings capable of such tasks.