St. Thomas Aquinas reflected on the meaning of leaving everything behind in order to follow Jesus, in the following excerpt from his commentary on the Gospel of St. Matthew.  Here, he examines how James and John responded and shows what the Christian must do.  In the course of this, he solves an apparent knot in the Gospels:

"They left their nets, boat, and father.  This signifies that we should leave all our worldly business behind for the sake of Christ, for which the net is a sign.  2 Timothy 2, 4: 'No one fighting for God enmeshes himself in worldly business', whether with riches or possessions, for which the boat is a sign.  Matthew 19, 20: 'If you wish to be perfect, sell all that you have and give to the poor, and you will have treasure in heaven, and come, follow me.'  The 'father' signifies carnal love.  Psalm 44, 11: 'Forget your people and your father's house.'  The name 'Zebedee' is interpreted, 'turbulent stream', which, in the spiritual sense, signifies the world.  But the question is this: It seems that these two sons sinned when they left behind their poor, elderly father because children were held to provide for their parents.  So, generally speaking, is it lawful for someone to leave their parents in their last necessity in order to enter religion?  The answer is that a counsel never trumps a commandment.  The commandment is, 'Honor your father and mother' (Exodus 20, 12).  Therefore, if a father is in no way able to live without the help of his child, the child should not enter religion.  But this was not the case with Zebedee because he was able to take care of himself and had the necessities of life.   

"There is also a question regarding the literal sense.  Matthew seems here to contradict John and Luke.  In John 1, 28, John says that these men were called near the Jordan.  Matthew says that they were called near the Sea of Galilee.  Also, in Luke 5, 10, Luke says that Jesus called Peter and Andrew and James and John at the same time, although no mention is made of the last two but it is believed that they were called there as well.  Again, it was said that they were called together, but here it says that they were called separately.  The answer is that the call of the Apostles was three-fold.  At first they were called only to familiarity with Christ.  This is described in John 1, during the first year of Christ's preaching.  This is not negated by what is said afterwards, that his 'disciples' went with him to Cana in Galilee (cf. John 2, 2).  According to Augustine, they were not yet 'disciples', but would be in the future.  It is the same case if we should say that 'the Apostle Paul' was born in Tarsus in Cilicia, when he would not yet have been an 'apostle'.  Or, John might be calling all those who believed in Christ, 'disciples'.  Then, secondly, they were called to discipleship, and this is described in Luke 5.  Then, thirdly, their call was to adhere completely to Christ, and this is what is described here.  According to Augustine, it is clear that in Luke 5, 11, the words, 'And having brought their ships to land" mean that they had a boat and were taking care of it as though they meant to return to it.  But here, Matthew says that they left behind 'their nets and their father', showing that this was the ultimate following."