The Reacher Classifications sum up my opinions on this. I think someone already put those up, but here they are again for completeness's sake.
1)Family History of Service.
I call this the Hadvar route because I'm a Skyrim fan and thats basically his backstory. Your father was a soldier, his father before him, and his father before. Your entire family has always been military, so you can't imagine another way to live. I respect those guys, if only because they do what they believe they were born to do.
2)Patriotism, and a drive to defend one's country.
They serve because they think they will be able to protect people by doing so. They accept that they may one day fall in battle, so that several people won't have to. To paraphrase Gavin Dunne (also known as Miracle of Sound): They accept that they may fall for the weak and the lost. The willingness to make that sacrifice if the need arises gets you a lot of points in my book. I probably respect this variant the most, even if their actual service never puts them in the place where such sacrifices are necessary. Its the thought that counts here.
3)Need a Job.
Fairly common here. They do what others do, but with the simple reason that they need a paycheck. Can't say I can dislike someone for going into the service because of that in this economy. As for my opinion on the kind of people... to paraphrase the Sniper: Army's a good job, mate. It's challenging work, out of doors. I guarantee you'll not go hungry. Cause at the end of the day, as long as there are two countries left on this planet, some government will want another gone. They are like police to me, just a job with decent benefits for what they do, and way too little pay if they are going into actual danger.
4)People who want a legal way to kill people.
They are Rabid Dogs to me. Might as well have them somewhere they have an off chance of doing something good in the world. Don't quite respect their motives, but I can respect their potential usefulness. It keeps the sociopaths in places where they can do some good for the world after all, or some evil depending on your politics.
Basically, if someone enters the army because they think that its a way to make the world a better place, they can have a spot in my hall of fame. If they do it for a job, its a good job, and they do something I sure don't want to. If they want to kill people, then at least we are keeping them pointed at something that is at least approaching being a threat to us.