There are many reasons why you cannot keep a deer as a pet. The main reason is that they are wild animals and not meant to be domesticated. Deer are also very shy creatures, so they would not do well in a household with children or other pets. They also require a lot of space to roam, and their diet consists mostly of plants and grasses. If you live in an area where deer populations are high, then it is also illegal to keep one as a pet.