In the comments on John 21: in "Biblical Perspective" Dr. Gayle Woods gives us answers to this question:
Often when we are at a loss to know what to do, we revert to what is most familiar. This was the case with the disciples. The natural leader of the group stated what he was going to do. The others who felt the need for close companionship with those who had faced the same experiences for the past three years stated that they wanted to go along for the trip. These men were professional fishermen and knew the best time and the best places to go to fish. Their hearts were not in their pastime, however, and they continued to work throughout the night without catching any fish. They were tired, frustrated men without a purpose.
Discussion: Why were these men without purpose?
Source: Miracles of Jesus: Adult Teacher's Insights, page 41.