10 month dog jumps every single stranger he sees. why?

he used to be off leash when we were outside and he loved playing with kids (really only chasing them lol) and people on bikes for about three months now we had to start putting him on a leash because he’ll hump people so much like they can’t come near him because he’ll try to hump them if we don’t push him in another direction. he always cries like screams whenever he seems another dog. why is he behaving like that? he’s a 10 month shih tzu
10 month dog jumps every single stranger he sees. why?
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