The point of the story of Mary and Martha is that Martha is so consumed with the minor details of life that she neglects the very important things that can more easily be put off. Specifically, she works hard to serve their guest Jesus while her sister Mary simply sits and listens to Jesus. Martha is understandably frustrated, but Jesus offers her this gentle rebuke:
Martha, Martha, you are worried and bothered about so many things; but only one thing is necessary, for Mary has chosen the good part, which shall not be taken away from her. (Luke 10:41-42; NASB)Now, Luke has positioned immediately before this brief report perhaps the most famous parable in the Gospels--that of the Good Samaritan. (Interestingly, this parable and the story about Mary and Martha appear only in the Third Gospel.) When the 'lawyer' asks Jesus, "Who is my neighbor?", Jesus responds with a parable highlighting that anyone, even a hated Samaritan, can prove to be a neighbor, and thus worthy of our love (Luke 10:27; cf. Lev. 19:18). Indeed, in the parable the Samaritan acted much more like a neighbor than the priest of Levite.
But, for a moment, I'd like to think about the priest and the Levite rather than the Samaritan; thus, the point I am making is not the main point that Jesus wanted to make. The text does not explore the motives behind the actions of the priest and Levite; it does not tell us why they passed by the half-dead man without helping him. However, if I may rely on personal experience/observation combined with the exegetical acumen of VeggieTales, I might say that a good guess as to why someone would ignore a man in need is because the would-be helper feels too busy with his own life.
In fact, in the VeggieTales version of the Good Samaritan, the 'priest' and the 'Levite' (actually, in this version, a mayor and a doctor, I think) pass by the man (or, cucumber) in need while singing, "I'm busy, busy, dreadfully busy, you've no idea what I've got to do; busy, busy, frightfully busy, much, much too busy for you."
Why do we often fail to help those who obviously have needs, even though we know that this is the type of thing we should do--I guess because we feel we haven't the time for such things, given the constant demands of life. We must get our child to piano practice right now, and thus we cannot stop to help the person whose car has stalled. We must do the minor details that feel so urgent, and thereby neglect the truly important things. Is this starting to sound like the story of Mary and Martha?
Reflecting in this way on the juxtaposition of the Parable of the Good Samaritan and the story of Mary and Martha might show that both stories are mutually illuminating, if not for Luke, then at least for his modern audience.