1 dog year is equal to the same as 7 human years because dogs have a shorter lifespan than humans do.
Most dogs only live 10 to 20 years at most.
Large dogs live shorter lifespans than smaller dogs in most cases and so when a dog especially a larger dog is 1 year old then they are about 7 years old in human years.
A 2 year old dog would be about 14 in human years and a 3 year old dog would be 21 years old in human years, a 4 year old dog would be 28 in human years, a 5 year old dog would be 35 years old in human years a 6 year old dog would be 42 years old in human years, a 7 year old dog would be 49 years old in human years, a 8 year old dog would be 55 years old in human years, a 9 year old dog would be 62 in human years, a 10 year old dog would be 68 in human years and so on and so forth.
Still though a year is a year but it's just a way we refer to dogs lifespans.
Dogs have a shorter lifespan than humans so it's a way to make humans feel better about their dog dying a shorter life.