Why do horses fight the bit?

Could you elaborate on the phenomenon of horses fighting the bit? It's intriguing to understand why these majestic creatures would resist such a fundamental aspect of riding and communication. Is it due to discomfort, pain, or perhaps a lack of understanding on the horse's part? Could it stem from training methods or the horse's personality? And how does this behavior affect both the horse and the rider? Understanding the root cause of this issue is crucial for promoting a harmonious and successful partnership between horse and rider.