Why do cats and dogs like being stroked?

Most pets liked being stroked, I do know a cat who hates it and runs a million miles every time I come near him but most cats like it!

Stroking a cat or dog releases a hormone called oxytocin. This hormone is actually released in both the owner and the animal, lowering blood pressure and reducing anxiety. Scientists assume that because these feelings are obviously pleasurable to both the owner and pet over time we've 'learnt' to continue. But why have we learnt? Scientists say that it's kind of like natural selection. This is because it's pretty much a huge survival advantage for many animals to be domesticated because not only we benefit from them but our pets get a free home, food and so on. It's this scientists believe is the reason why animals love us stroking them - a feeling which is gotten stronger over time mirroring a much smaller scale of evolution. Now, we've reached a stage where they (well most of them :) ) enjoy our company and vice versa. 

Can animals be allergic to humans?

Yes, they can. Scientist's don't know much about allergies with wild animals but it has been known that dogs and cats develop allergies to their owners. Like when humans are allergic to animals, the main culprit is the dander or dandruff that falls of us. Around us everyday there's dust that contains varying amounts of dead skin. When cats are allergic they show symptoms such as coughing, wheezing and shortness of breath. Dogs show symptoms such as itching, sores and skin infections. This problem is increasing a lot because we are steadily domesticating our pets where they are continuously exposed to human allergens.