No offence to anybody but this, as a topic, is asking for some really lame answers. What's something you never believed until you experienced it? Because absolutely anything and everything would be a correct answer people go with their favourites.
Which is fair enough. Its fun to have an excuse to right about how you fell in love (and you can't tell me that isn't the most popular answer going) when you weren't expecting it. Or share your freaky stories about ghosts/shadows/voices/paranormal activities.
Which just makes it all the harder to have a truly unique perspective on the question at hand - and I'm a big fan of unique. But I will endeavour to avoid 'woe is me' tales of heartbreak. Instead I'm going to have my say about military indoctrination.

The military I have to say is a very sly beast indeed. It has, without a doubt, perfected the art of indoctrinating its personnel. Members who have been in a while warn you about it, before you get in, as soon as you are in, after you've been there 6 months. Your response is that you aren't indoctrinated yet - you are exactly the person you were before you joined.
Yeah... Right... Keep telling yourself buddy. Like you didn't have the phone call on day one when you had to explain out every other word out of your mouth to your mother/father/boyfriend/loved one - whoever you were lucky enough to have 5 minutes for.
Like you didn't have to force-ably stop yourself from marching the first time you were out in public after you joined. Or get the insane urge to stop that hippy who just walked past you and take him to the barber for a real haircut.
Suddenly, one day you are talking to one of your old friends from home and you've just had to explain what a drill square is for the 17th time and it hits you. Civvies are morons. There is nothing complicated about a drill square. Hell, you'd rather be marching around it a few times than eating rat pack and playing army (navy doesn't like playing army much). Its at that point you realise just how indoctrinated you are.
From then on you try and play it down. You avoid shaving on weekends. You make a concerted effort not to mention being in the military to people you met. You start buying clothes that wouldn't be allowed in the mess to wear on weekends. And still you slip up. You use that acronym without thinking. You use slang that is just a daily part of your vocabulary. You go out with your mates and keep even 75cm strides.
I never thought it was going to happen to me. When I joined up (and it was only 3 years ago) I figured that you had a choice in the matter. That you could dress like a sailor, talk like a sailor, work like a sailor and somehow not think like a sailor. I thought I'd still be the same me I always was. I'm not. For as long as I live there will always be a part of me which is military.