我认为狗狗吃苹果会吐的原因可能是因为苹果中含有的膳食纤维过多，这就导致了狗狗的胃肠道负担增大，从而引发呕吐。另外，dog's digestive system is not very efficient in breaking down and absorbing dietary fiber, so eating too much of it may lead to gastrointestinal discomfort, including vomiting and diarrhea. It's also possible that dogs can be allergic to apples, which could cause vomiting symptoms.

If a dog has never had an apple before but vomits after eating one, it's recommended to stop feeding them apples and consult a veterinarian to rule out any potential allergy reasons. In general, eating apples might cause vomiting in dogs but this isn't the case for all dogs. If your dog vomits after eating an apple, you should seek medical attention immediately and avoid giving them apples again in the future.