Shiner in his dissertation notes one peculiarity of the Markan call story in 1:16-20.
There is a lack of motivation. In other words, the disciples are not given a reason to follow Jesus in the narrative.
The typical pattern of call stories are to first expound the values of a life of wisdom or demonstrate the power or superiority of a teacher, but in Mark there is no motivation provided for the fisherman’s decision to follow Jesus. The four fisherman do not appear to have heard Jesus’ preaching.
Shiner says that this lack of causation on the narrative level leads the listener to find causation on the discourse level. The hearer knows that the disciples respond to Jesus because he is the Son of God. Therefore, “the lack of psychological motivation serves to emphasize the more essential, spiritual motivation.”
But the lack of motivation also also serves to generalize the call stories and thus facilitate the hearer’s identification with the disciples. The succinct narration of the stories allow the listener to experience the disciples’ call as their own call.
The lack of motivation in the narrative universalizes the motivation at the discourse level.
Whitney Shiner, Follow Me! Disciples in Markan Rhetoric, SBL Dissertation Series 145 (Atlanta, Ga: Scholars Press, 1995), 185–6.