Dogs have short lifespans primarily because their bodies age much faster than humans. This accelerated aging is due to their higher metabolic rates and genetic programming, which causes their cells to divide and age more quickly. Dogs' life stages happen at an accelerated pace—for example, a 1-year-old dog is roughly equivalent to a teenage human, and by age two, dogs have reached full maturity, a stage humans reach in their mid-20s. This fast life cycle results in overall shorter lifespans for dogs compared to humans. Additionally, size plays a key role: smaller breeds tend to live longer, often 15-17 years, while large breeds like Great Danes typically live only 7-8 years. Larger dogs age more rapidly due to the physical stress of rapid growth and are more susceptible to age-related diseases. Genetics and selective breeding also influence lifespan, with certain breeds predisposed to genetic health issues. Environmental factors, such as diet, exercise, veterinary care, and living conditions, can impact how long a dog lives, but cannot fully counterbalance their inherently faster aging process. In brief, dogs live shorter lives because their entire biology operates on a faster timeline than humans'.