Problem Solving-Best Employee has negative attitude.

One of best employee has negative attitude. He always says about the new projects ,that this can not be done. We try to explain all technical points , but he does not change his opinion.
I invite suggestions from all readers to suggest how to solve this problem ,so that the attitude can become positive. Can we analyze that this problem is personal,official or mental. Is it a problem about the salary. Is it a negative attitude.
problem.