why is the relative ease of dehydration of alcohols tertiary>secondary>primary..?

The order of ease of dehydration of alcohols is :

Tertiary alcohols>secondary alcohols>primary alcohols. This order is on the basis of reactivity of the intermediate carbocation. We know that, tertiary alcohols will form tertiary carbocation, while secondary alcohols will form secondary carbocation and primary alcohols will form primary carbocation. The order of stability of the carbocation is : tertiary>secondary>primary.

Thus, tertiary carbocation will be formed easily and primary carbocation, will not be formed easily. Therefore, the alcohol furnishing tertiary carbocation will undergo dehydration very easily.