Lots of them want glory, power, to be able to kill people, and many do it just because they don't think they have any other options.
Hmmm, I joined because I didn't want to kill anyone (or have anyone get killed). I believe in deterence. I enlisted in the Army Reserves in High School and then between my sophmore and junior years got an ROTC scholarship (Yep, carried a full class load, Computer Science + ROTC classes/events at another school and worked full time.)
The US Army in it's infinite wisdom decided that even though I had a CS degree and had put Combat Arms (Cavalry, Armor, Artillery, Infantry, Engineers, Aviation) on my dream sheet, I ended up as MP Officer.
Most of my troops joined for the following reason: They wanted to be cops, deputies or state troopers. If they joined the army they got the following to put on their "I wanna be a cop" application:
1. Four years of LE experience. (Even though we did ~4-6 weeks of actual Garrison Law Enforcement per year. The rest of the time we were in the field playing motorized infantry or expendable/replaceable scouts.)
2. They all took CJ classes either on post or via mail. (most got their Associates, a few even got Bachelor's.)
3. They got the Veterans Perfernce Points you get when applying for government jobs.
So no, they really didn't think the didn't have "other options". They were actually pretty smart and had a plan to achieve their goals.
Now yes, there are some people who joined to get away from bad situations. My first Platoon Sergeant had joined in the mid-60's because a judge told him "Go to War or Go to Jail".
However, it doesn't suddenly make someone understand the motives and decisions behind war any better, it just means they know what its like to sit in the sand in a country where lots of people hate you and you get shot at, etc.