Here's why:

* Nutritional Dependence: Parasites often lack the necessary enzymes or organs to obtain their own food. They rely on their host for nutrients, either by consuming their host's tissues, blood, or bodily fluids, or by absorbing nutrients directly from their host's body.

* Shelter and Protection: Many parasites, particularly those with simpler body plans like tapeworms or protozoa, need the protection and shelter provided by their host's body. This allows them to avoid predators and harsh environmental conditions.

* Reproduction: Some parasites require specific conditions within their host's body to complete their reproductive cycle. For example, some parasites need to be in a specific stage of development within a host's digestive system before they can release their eggs.

Essentially, parasites have evolved to exploit the resources and environment provided by their hosts. This dependence is a crucial part of their survival strategy.

It's important to note that parasitism exists on a spectrum. Some parasites have a minimal impact on their hosts, while others can cause significant harm or even death. However, all parasites share the fundamental characteristic of relying on other organisms for survival.