What are the reasons that a dog can be afraid of someone?

I have been walking my neighbors dog for over a year. I usually walk it twice a week. Just recently, when I try to take it out, it barks and growls excessively. Last week, when I tried taking it out, it bit me. When he barks, it is not just a normal bark, it is a scary type of bark. I tried everything. I waited there for the dog to calm down, tried giving him a treat, and using a spray bottle (which the owner suggested). Today, when I tried taking him out, he was going crazy and then went back in his cage. Personally, it scares me to walk him and when i try to tell the owner, she doesn't understand because when she sees the dog, he is perfectly happy. I'm not sure why this is happening since i have known this dog for a really long time. I go by myself and i haven't changed my perfume or anything recently. If anyone could help me with this problem, i would be much appreciative. I don't have a dog myself so I don't know if this is a normal occurance. But, I really need help. Thank you!

Do you have cats? Or some other animal you come into contact with? Dogs can smell other animals on you and may recognise you as prey so this could be it... Otherwise I don't know sorry... maybe you should take it to the vets?

Of course, there are numerous reasons:• Past Experiences: Some dogs can fear people of a particular race, gender or by some other physical factor just because, in the past this dog was abused or had a bad experience with this person. Some dogs hate people who wear hats, dark skinned or light skinned people, or men, just because they've come to fear them through some possible pass trauma. This can be the same for certain places or areas, that bring back past memories. A walk in the park for one dog could be a reminder of a past attack by another dog he met at the park..ect• Fear Driven Aggression or "Lack of Socialization": Some dogs fear what they don't know and this is usually due to a lack of socialization on their owners part. Dogs at a young age should be taught and desensitized around people, places and other animals, so that they're not so fearful and anxious about everything. • Good Reason: Dogs are living organisms and can become fearful just based on suspicion or just innate instinct. Some breeds of dogs are more prone to being wary of strangers then other breeds. Some breeds of dogs, especially those bred for guarding were bred to be wary of strangers and very in-tolerate of those they don't know. Some of the breeds are Rottweilers, Chow Chow, Mastiff Breeds, and some smaller dogs like Terriers. Some dogs don't like objects, people carrying this that can possible hurt them or people who approach them to boldly. It can be the things I listed or something completely otherwise but there are tons of reasons why dogs can be fearful.