Seriously, have you explained to your parents exactly what it is you want to do and why...ie your dream? For instance, Naval Aviation, Intel, or whatever it is you're entering in order to do. Also, explain to them the current situation of the economy and the job market...let them know that you won't have to worry about job security. Not to mention all of the possible education benefits for furthering yourself by attending grad school (Tuition assistance, GI Bill, Naval Post-Grad School etc.), which can keep you from being in debt for life.
You could also mention the travel and the military discipline/lifestyle if these are also valid reasons for your joining. I mean, really I think it's silly the way some people react with 'but why' when you tell them you joined the military...don't they know that there's more to life than money. Yes, it's nice and you're probably not going to get rich, but sometimes people like to do something honorable and be a part of something much greater than themselves.
JR