Why are people scared of a big dog? even if I tell them shes nice?

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My moms neighbor the other dy about had a cow when she seen my dog the other day when we were at my moms, My boyfriend was out walking her and the lady seen her she freaked and asked him if she was mean, he told her no shes just a big puppy (dont you think if she was mean we would say she was since we wouldnt want her biting someone?) then when I took her out the next time she did the same thing only she had a cow and was like "You have a good grip on that dog dont you?" No lady im holding the leash for fun I figured Id let her go whenever she feels like it.....omg.....Shes a bulloxer (American bulldog boxer mix) she looks more like the boxer she has her tail and he ears so she doesn't even look tough. you can tell a mean dog from a nice dog usually cause if thee friendly like mine she will just want to lick and jump and play but mean ones are skiddish and not so much play full right off the bat.this lady has a little dog and I think shes just to used to having a little dog and not a big one. but just because a dog is big doesn't make them mean.