How to get my dog to be less aggressive to other dogs?

I just got back from taking my dog for a walk. her name is Sadie, and she is a golden lab. she is very friendly and loving (I'm not just saying this.). we had another dog who recently ped away and he was quite smaller. she was always like a sister to him, always sharing the same space in kennels and sharing food and bedding. she is better with littler dog's than larger. she was best friend's with him and they never fought or growled at eachother. it's only stranger dog's. today I was taking her for a walk up the street (I rarely do this, but today I didn't have my car to take her to a hiking trail) and we saw a little girl walking with her mom. she asked if she could pet my dog and I said "yes, she's friendly." and the little girl came up and started giving Sadie big hugs and playing with her tail and Sadie licked the little girls face and wagged her tail. the mom and I talked about the breed of Sadie. eventually they continued walking and I started back up the hill. we came to their unattended yard where her 3 dog's were left in the front yard. i'm pretty sure there were 2 chihuahuas and 1 medium mix. the medium one bounded up and got into Sadie's face so she gave a growl. I tried to separate them but it's hard when my dog was on an extendable leash and their dog only wore a chain. the dog again tried to come in and Sadie would growl and it would back away. the mom came running up and grabbed her dog and I said "I'm so sorry." and she said "no, no, it's okay, I'm glad your dog could show who was boss." and we continued on our walk. Sadie was perfectly fine, her and the other dog didn't make any real contact. I still feel ashamed, the other dog was just trying to be friendly. when other dog's bark, Sadie doesn't bark back, which I find good for our walk back and the other dog's that were now on chains were barking. so, your opinion? and how do I get my dog to be kinder to stranger dog's?