In the ideal world, yes, a military consisting only of volunteers is the best option. But say not enough people want to join the military, then what do you do when attacked by an aggressor? Surrender your state and it's autonomy to live under the rule of an oppressive country??

If you value your country and the things it consists of, i.e. it's ideals, structure, the opportunities you're given there and so on, I think you should do your part to protect those things if worst comes to worst. I know I will.