The reason why hermit crabs live in snail shells is because of the following main reasons:

1. Defense Mechanism:
   The hermit crab’s abdomen is very soft and fragile, making it vulnerable to predators. They don’t have a hard shell to protect themselves like most crabs. Therefore, the snail shell provides the necessary protection to allow the hermit crab to quickly retract into the shell when threatened, avoiding predators.
2. Adaptive body type:
   The body shape of the hermit crab, especially the abdomen, is curved and flexible, able to adapt to the spatial structure inside the shell, can coil inside the shell, and uses the fan tail to hook the top of the shell to ensure that it does not slide out of the shell when moving.
3. Growth needs:
   As hermit crabs grow, they need to change their shells to accommodate their growing size. Hermit crabs will choose different sizes of snail shells as temporary shelter until they find a new shell that is more suitable.
4. Symbiotic relationships:
   Hermit crabs sometimes form symbiotic relationships with organisms such as sea anemones, which attach to their shells and provide an additional defensive barrier with their own spike cells, helping to protect hermit crabs from potential predators.

In conclusion, hermit crabs inhabit snail shells for survival and self-protection, which is a natural selection and evolutionary strategy for their own physiological characteristics. At the same time, this behavior is a vivid example of the phenomenon of biological adaptation and symbiosis in marine ecosystems.