Everyone joins thinking they know what they're letting themselves in for, but being in the military is a completely different lifestyle to what you'd have experienced up until now. It's more immersive than just about any civilian job you can name and for extended periods of time it's all you do for 24 hours a day, 7 days a week. IME it also bears very little similarity to what TV and the movies would have you believe.
You've no doubt been making assumptions about military life based on the big differences between how you live now and how you'll live then, but it's the small stuff that you'll end up caught by. I'm not going to tell you "they'll beat it out of you", because that sort of thing is (or at least should be) frowned upon, but you will find a military life is different than you expect in a hundred tiny ways. If you're still in in 5 years you'll look back on yourself as you are now and shake your head at your naivety, although half of that will be due to 28-year old you laughing at 23-year old you no matter what line of work you end up in. Even earlier than that, by half-way through basic training you'll probably find yourself sniggering at some of your more starry-eyed expectations, and if/when you make it through basic the memory of telling people "I am in the U.S.A. Military Navy" before you actually set foot in training and expecting them to take you seriously will have you rolling around the floor in fits of laughter.