He is not sure why, but he gets a hard on every time he puts on the jock and cup.  My guess is the combination of his junk being contained while is ass is exposed and the full contact sports with his friends and teammates.

Soon he will be wearing jockstraps all the time, because they feel right and keep him constantly horny.  Eventually one, or more, of his friends will take note of his raging hard on in the locker room and turn him out.  The jock straps giving his former friends easy access to his cunt.