If you enlist, the military will be part of your life for at least eight years, including time in the Reserves. A lot could happen during those years. The United States is currently engaged in combat in four countries (Afghanistan, Pakistan, Iraq, and Libya). If you're in the Reserves, the military could call you away from your home, job, or school.
What if you don't like military life? You can't just quit, and the military considers it a crime for you to leave your unit or disobey an order. A recruiter may tell you that you can give the military a try and automatically get out after six months if you don't like it. This is not true.
There is no such thing as a period of adjustment in the military during which you can just opt out.