All sorts of reasons, I'd imagine.
But I know that in some families it's almost like tradition. I guess if you've got ancestors who had successful service careers and are celebrated for it then you might want to follow in their footsteps.
A more practical reason might be financial. I have a relative who joined the navy right out of college. He was immediately earning a good salary which he couldn't spend because he was on a ship a lot of the time and all his living expenses were paid so by the time he got out he'd saved a big chunk of money. That beats going to university and getting 30k in the hole before you've earned a penny.
But that was in a time of relative peace so he wasn't in much danger of being deployed. I don't think you could say that now.
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I do apologise. But also I can't promise it won't happen again.