It's basically writing about a subject you strongly believe in, and showing your personal example on why it holds true. If something happened to your grandfather, or yourself, or even a friend, and it relates to proving the message you're trying to send, you use that as an example on why you believe this. It's pretty simple, what's probably the most frustrating part is the time it takes to write it out. With your given question, what is your dream? Why is this your dream, what personal aspect of your life made you want this. Say your dream is to be in the army... Why do you want to be in the army? Is it to make your family proud, to defend our country, to continue to include your bloodline in the military, is it to honor someone close to you that was in the military and as passed? The problem people have with personal narratives is over complicating it, when it's simple.