"I should like to learn alchemy, if you are willing to entertain another student sometime," he says. Mostly he would like to learn alchemy because then he gets to spend more time with Mercutio, but they're friends, that's normal. It's normal to want to spend time with your friends. Though -- damn, what if this Claudius just keeps making comments insinuating Benvolio has feelings for Mercutio while they're trying to learn alchemy? He should have kept his mouth shut.