The body speaks up to the lord of the people who has ridden it all his life, reminding him that it has always carried him faithfully and never balked before, which is the animal nature protesting that its sudden refusal means something real is in the road. The instinctual sense the flesh carries, the organ meant to feel what the reasoning man cannot, has been reliable from the beginning, and when it finally shies it is because a genuine obstacle stands where the mind sees nothing. A person who has let that inner sense go dull through long neglect will argue with his own body instead of trusting the warning it gives.
The exchange is the conflict between calculating mind and bodily instinct, told as talk between rider and mount. The flesh knows first and speaks plainly, and the man's grudging Nay admits its whole history of service. The meaning is simple: the body's sudden refusal is a signal worth reading, not a fault to be beaten.
Ass: the bodily nature and its instinctual sense, the mount that has carried a person faithfully and now warns him