Along with broad shoulders, a narrow waist, a symmetrical face and a square face, a man bun signals high levels of testosterone, which some women find attractive. See How can a man tell if he has high testosterone
People, especially women, find man buns attractive because it's not very common to find a guy with long hair and a bun. See Why novel and bizarre things catch our attention
Some women are attracted to hairy men, especially men with long hair, which is why they find man buns sexy. See What do women find attractive in men
Some people find long hair on men extremely attractive, especially since many celebrities sometimes have long hair.
Many people find man buns attractive because many celebrities do it such as Jared Leto, Brad Pitt and Bradley Cooper.
Since it's not very common for men to have buns, those who have them send a message that they don't conform to social norms, which some people find attractive.
Man buns indicate healthiness and youth since a head full of hair means the man is staying healthy.
Some women find men who take care of their looks and hair attractive.
Some women think man buns are a sign of ruggedness and roughness, which they find attractive.
Pulling the hair back in a bun makes the face look more defined and attractive.
Some women like trendy men who follow the latest fashionable hairstyles, which makes them like men with buns.
Some people think of men with buns as confident, secure and manly, as they don't have a problem sporting a style that is generally worn by girls.
Man buns are believed to be attractive as they make the man stand out in a crowd and have all eyes on him.