The need for excess acid in dehydration of alcohols
Alcohols generally are very poor acids and have pka's in the range of 16-17.

Hence, if the medium is only slightly acidic, only some alcohols will protonate, after which the unprotonated oxygen (of ROH) will attack the generated carbocation, following which the H+ ion will leave making H+ act as a catalyst, resulting in the formation of ether.

But, if the solution is highly acidic, we would be generating the carbocation in full and there would be no nucleophile left to attack the carbocation.

We need to maintain the optimum pH as substitution reactions are generally favoured over elimination reactions especially in the case of primary and secondary alcohols.