Why do dogs breathe faster then humans?

Because dogs have more hair than humans, so they get hotter and breathe faster.

Additional information:

It's unclear whether you're asking about dogs panting or simply breathing faster in general.

Cooling

Unlike humans, dogs have very few sweat glands (most are in their paw pads), so they cool themselves primarily by panting. This allows air to travel across the moist lung tissue, which helps the dog cool down.

Breathing, in General

Smaller animals have faster respiratory and heart rates so their bodies can take in enough oxygen. Blood travels through their circulatory system much faster than it does through a larger creature's system, so they have to breathe more often.