It's mainly territorial. The rooster is announcing to the world that the space he is in is HIS and his only. It's like he's saying, "Hey, this is my space!" There are other reasons suggested as to why they crow as well, such as in response to being scared or when their hen cackles after laying an egg.
Did they grow up together? They have established a hierarchy (aka "pecking order"). The dominant rooster will most likely do the most of the crowing and breeding with the hens.