Why do heroes in the Water Margin like to use "brothers" to match?

Equivalent to today's brother's name. In the Water Margin, everyone becomes brothers because they are brothers, so why flesh and blood? For men who are eager for true feelings, they will naturally regard people around them as brothers and sisters, and call older and prestigious people brothers. In fact, they are all brothers! For example, Sun Erniang called Song Wu "my brother" and Chai Jin called Song Jiang "big brother". At that time, this kind of address may have been the envy of those literati who killed dogs, but as an arrogant professional, it was too humiliating to imitate other people's address, so I passed it on. In fact, if a scholar calls Song Jiang, he must call him a brother, so jy and others call him a brother. One is stupid, and the other is obvious imitation, so he simply calls him a brother.