Think about Edward Elric and headcanons related to his hair.
He's not normally a sentimental person, but he keeps his hair long in a "hair holds memory" kind of way. The last thing he had to hold onto his mom after they burned the house down was her brushing his hair. So while he denies that as the reason—saying he just can't be bothered instead—that's why he lets it grow long in the first place.
So as the years go by it just becomes a Thing. Even if there are things he'd like to forget he refuses to just like he refuses to cut off his long hair. The memories fade a little at a time just like how his hair is only trimmed a bit at a time every few months. And only loved ones like Izumi, Al, Winry and Roy are allowed to trim his hair (he trims his own bangs usually cause they annoy him faster).
His hairstyles depend on the setting. For professional and special events he wears a ponytail because he associates it with looking older and a little bit classier (this totally isn't because it makes him look more like his old man /sar). For everyday stuff he wears his braid because ponytails give him headaches after awhile. As he gets older and his hair is even longer he starts to wear it sorta like his mom, where its pulled to the side loosely but ends in a braid instead of a small tail. Except for bed and bathing he never wears it down because there's so much of it and it gets in the way.