We've got a 3 year old border collie. We've had her since 9 weeks and have a strong bond.
As a pup, she loved both dogs and humans. As she's got older, we've found she's less tolerant with other dogs, especially pups or smaller dogs, while remaining calm and friendly with humans.
It's not every dog, she has "mates" down the Park and other dogs shed ok with. I'd say it's about 1 in 3 she has problems with. We noticed she was baring teeth with others, now she's getting snappy.
It's so unlike her, she's normally chilled and calm!
Any advice welcome xxx