The objection was that this was done on the Sabbath day, in which case plucking grain was perceived as harvesting, therefore a work and thus, a violation of the Sabbath law. Since the disciples were followers of Jesus, the Pharisees very naturally (and rightly) assumed that their activity met with their Master's approval. Jesus then was the focus of their criticism.

But if these are violating the law, then what of David, who with his soldiers when fleeing from Saul took and ate of the showbread in the house of God (cf, 1Sa.21:1-6). The consecrated bread was to be eaten by the priests only (Lev.24:5-9), yet David when hungry and in need allowed and demanded this exception from the priest and that on the Sabbath day. So Jesus' opponents are faced with a dilemma: they must choose between their traditions and interpretations of the law on the one hand and David their great king on the other. But in opting for David, they would thereby acquit the activities of Jesus' disciples whom they have already pronounced guilty, and implicitly acknowledge the narrowness of their own minds.

David needed to eat, and so now do Jesus' disciples. The underlying assumption seems to be that of human need which takes precedence over religious ritual. This understanding fits well with v.27: "The Sabbath was made for man, and not man for the Sabbath." That is, this religious observance was intended for man's benefit, not his enslavement.

Their interpretation of the law was very rigid and became an end in itself. It left no room for compassion or any act of goodness which conflicted with it. It offered no governing principles to guide life, only regulations. When law becomes the focus and when human traditions are given the status of divine mandate, the spirit of Christ is absent. The gospel of Christ offers a rest which is more than physical and temporal but, in him, spiritual and eternal. 

Some people are so cautious of not doing big work during Sunday, but failed to go the Church and attend mass. And if they attended the mass they avoid occasion to do apostolate or acts of charity to the needy. Sunday is not only a moment to rest, but it could also be an occasion to put into concrete action what they learned from the holy mass.

People are more important than systems and programs. People are more important than rituals and religion. John wrote that if a person loves God, then that person will love his brother (1 John 4:20). William Barclay wrote: “The best way to worship God is to help men.” It might be easy to think that loving God and loving one’s neighbor are two different things. They are not. Our love for God is expressed precisely in how we treat others. If we are mean, hateful, cruel and inconsiderate of others, that is a demonstration of how devoted (or not devoted) we are to God.