So when I walk with my dog he'll play with other dogs just fine as long as this is not happening near me. When they are near me, my dog will try to jump for the other dog's neck (I have a Yorkie) and bark. I don't think he's trying to harm because he's so tiny but why is he doing this?
He views you as being "his" human, and he's trying to ensure that no other dog tries to get too close to you. He doesn't want to share, and is also probably trying to protect you.
To me it sounds like over protective and possessive behavior. I'd try to stop it now before it becomes worse which is very likely to happen if you don't let the dog know this is not acceptable.