Why do dogs go crazy when they see each other on the street ?

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I always wonder why do dogs act like that ? Perhaps it is their animal instinct ? Perhaps, they are really interesting in each other ? Perhaps, they are excited because they meet new friends ? Perhaps they don't feel safe when other dogs around ? Perhaps they feel the other dogs are threatening their territories ? Perhaps..... Frankly speaking, i don't know because I never have a dog as my pet even I did play around with the dogs of my friends a few time. If you know something about this issue, share with us ?