Backyard Poultry magazine had an excellent article on rooster's crowing. Unfortunately I just checked and it's not one of their articles that you can read online.
It's basically said roosters crow to mark their territory, to send out a challenge to other roosters that might be around, to call to their hens and just because the rooster is having a good day and feels like crowing about it.
I feel my rooster crows cause he is happy with the attention I give him..It is always the same red roo that crows too..now I heard something today that sounded sick from one of the younger ones..I just love to listen to them crow though..brings me back to when I lived on farm as a kid many many many years ago..
I can definitely tell the differences in my roosters' crows. Sometimes if I put them away early in the afternoon they'll protest and start crowing. Sometimes when one roosters crows, another one will charge at him to as if saying "I don't think so!"