They say that for every human year a dog has seven. Why is this?

The equivalence of dog years to human years varies depending on the size and breed of dog. Larger breeds tend to mature quicker and live shorter lives than smaller breeds. Also humans develop relatively slowly compared to other mammals, for example many breeds of dog will have reached full growth and sexual maturity by one year, which is roughly equivalent to 12-15 in human years. The second year for a dog can range from 3-8 human years with each year thereafter being roughly equivalent to 4-5 human years, but even this is inaccurate and can only ever be an approximation.

Rik Sargent, Science Museum