This may be reviving an old thread, but I will Rusk the ridicule.
I was being thrown many options of what to do with my life - acting, science, etc. I decided to go with the military for numerous reasons. I had many people express their concern for me, stating things along the lines of "it isn't Canadian to be in the Army", "you could die", "there are better jobs for you to attend". A background on my town is it's a tight town of 3k people. Highly pacifist as most of the town is filled with old draft dodgers and Dukhobors.
The only party you need to concern yourself with "convicing" (and it won't take a lot of effort, just the truth) is your own family. They may resent the decision at first, but if you can show that with an unwavering resolve, you can be a man and make your own decisions, your family will respect you for that. If your friends get angry and hate you for your decision (a thought that has maybe crossed your mind), then think of it this way - they aren't real friends if they are not willing to support you. People that "don't want to lose you" despite your desire to join are being selfish. Point that out to them if need be.
Look at why you want to join (doing "your part", being part of a bigger thing, heck even job security) and then what people may or may not throw at you as reasons not to join (you will die, dead end job with no skillful future, etc) and compare. If your list comes out bigger (and I will bet my two rubles that it will) then that will be your way to go.
Don't let your resolve change throughout your initial training though. It will get tough. There will be a lot of pain, sweat, blood, and tears, but keep your resolve. Nothing boosts their side of the argument when you call from training school complaining about life in the army. You will just set yourself up with counter-lines like "I told you so" and "What'd I tell you?".
The part you will have to do a gut check for is if your are ever wounded, will your resolve stay or change. That's what I have and I have had a year of "this is your second chance...", "we don't want to see you hurt again.." and all I can say is that those lines have a whole lot more substance than the ones you get when you join up.
In conclusion, you are your own man - do what you may and the people who really support you will follow. Make good with your family because you will definitely complain to them at least once throughout your first contract. Finally, always remind yourself what were the initial reasons for joining in the first place.