Which factor does NOT contribute to pulp irritation?

Excessive hydration is not a factor that contributes to pulp irritation. Pulp irritation generally arises from factors that either directly damage the pulp tissue or lead to the exposure of the pulp to irritants.

Cavity preparation, decay, and mechanical trauma all involve processes or conditions that can create inflammation or irritation within the dental pulp. For instance, cavity preparation can expose dentin and potentially irritate pulp nerves, while decay leads to bacterial invasion and further contributes to inflammation. Mechanical trauma can directly damage the pulp or cause microfractures, also resulting in irritation.

In contrast, excessive hydration specifically refers to an overload of moisture that does not have a direct pathological impact on the pulp itself, and thus does not induce irritation. This differentiates it from the other options, all of which are associated with physiological responses leading to pulp irritation.