This, Not That

Medium Matt is a bit odd. He likes only certain things. Can you figure out why he likes what he does from the following clues?

✦ He likes gum, but not mints.

✦ He likes the beach, but not the ocean.

✦ He likes cotton, but not wool.

✦ He likes medicine, but not doctors.

✦ He likes crystal, but not quartz.