Horses want to colic.
Cows get mastitis.
And sheep are just looking for excuses to die.
So true. Sometimes I think they do it on purpose to make us prove we love them and are prepared to spend enormous amounts of money on them. You'll notice they always pull this kind of thing when the owner is broke: it means she has to make a bigger financial sacrifice for the horse, proving she loves him even more. I have spoken to my horse about this and warned her that I do not have money for major veterinary surgery so she had just better not entertain any ideas about developing a major illness.