First:
Trevor, you're a good guy and a true friend!
Secondly:
Vaccines do not confer lifelong immunity. Mumps is a very easy disease if you catch it as a child (I know, for I've had it and one of my kids did as well). But if you catch it as an adult, it can be quite difficult. Sadly, since the MMR vaccine does not confer lifelong immunity, you are much more likely to catch it as an adult and suffer the repercussions of it. Had your friend had mumps as a child, he wouldn't have developed the disease, since once you have mumps, you have lifelong immunity. Sadly, not so with the vaccine, so you can thank your body for having strong immunity.