Why does your Jack Russell vomit so much?

Dogs vomit because they can. Mechanically it is easier for dogs to vomit than it is for humans so they do it more often and usually with less serious cause. Dogs vomit when they are excited, hungry, carrying for or around puppies, just drank too much water, ate something that didn't quite agree with them, and yes, also when they are sick. If the vomit appears normal, ie it is yellow, yellow and foamy, white and foamy, mostly water, or food, then it is probably okay so long as he does it occasionally (once a week or less). If he vomits often, it may indicate Allergies. Consult your vet. If he vomits something abnormal, ie something dark, something that looks like coffee grounds, worms, blood, or something else you cannot identify, then he needs urgent veterinary care. If it's just yellow or foamy, then it's probably because his stomach is empty, so increase the frequency of his meals or serve a snack between meals to keep a little something in his stomach and keep it settled. If it's very watery, then he just drank a whole bunch of water. They're more likely to do that when they drink after exercise. That's not good for him, so monitor his drinking when he is hot. It's the fast drinking that usually causes it so you need to slow him down. Let him have a short drink (a few laps), then wait a minute, a short drink, wait a minute, and so on until his thirst is quenched.