What is the reason for the obligatory nature of following a mujtahid?

Answer:

In all matters that one does not know, a person naturally turns to someone who is knowledgeable and aware. For example, a patient turns to a doctor. In religious matters that are of greater importance, one should also - if one is not a mujtahid himself - turn to a mujtahid who is an expert in the science of jurisprudence and religious rulings.