Paying attention to what is going on when your pets are happy or upset will help you to begin to know your pets better. Does your cat always run under the bed when someone arrives, or only when children come over? What to do with this knowledge is where I come in. Learning about how a pet's nutrition, illnesses, past injuries, and home life can affect thier behavior is one way to learn more about why your pet may be acting a certain way. A big reason why Mercy does not like active play with Ranger is because she had hip surgery when she was young, and has some arthritis now that she is older ( 14 yrs). We give her joint supplements and pain medication to help her. It is not likely she will want to play hard because of her hip, and knowing that helps us to understand her behavior towards a young cat like Ranger.
Your veterinarian needs to know your pet also. We can not know them as well as you can since we do not live with them. We can learn from owners what your pet's likes and dislikes are, how they are behaving and use that information to help diagnose health problems that may be the root of the behavior. So, the better you know your pet, the better veterinarians can help.