Our physical body is constantly exposed to all sorts of diseases and bacteria. No matter how careful we are we shall not have full control of the cleanness of the food and drink we intake daily. Several hours of flying on a plane we share and recycle the same volume of air with the other hundred passengers. We may wash our hands properly but we have no control over other people's hygiene and in the name of courtesy or politeness we risk being contaminated when we shake hands or hug someone. Our way of life and our social life opens the door for bacteria to creep into our physical body. This kind of external uncleanness was what the opponents of Jesus were worried about. They were much more concerned about the ritual defilement and ignored the internal cleanness. For Jesus both the external and internal cleanness are important; the former is good for our physical body and the latter is needed for our spiritual worthiness, the spiritual one. Jesus talked about the source of true defilement - evil desires which come from inside a person's innermost being. Jesus talks about the conditions of the heart. Whether our heart clean or unclean that it all depends on what we feed our heart with. A clean heart and pure heart is fed with God's love and his teaching while an impure heart is being fed with things that don't come from God and that heart is being contaminated and it will be a shelter for evil desires to dwell. In condemning the traditions of the elders Jesus rejects not their significant teaching but He dismisses the interpretation the Scribes and Pharisees made of the traditions.
Everybody knows that good quality food consumption makes our body healthy and junk food is not good for our body but none of them is hurting our spirituality. Knowing the side effects of the food intake for our physical body we should be careful of the food that effects for our spiritual life. We should be careful of what to read and what to see and hear otherwise we make our life miserable and the internal corruption and evil behaviour in us would cause tremendous harm for our love ones and for the others at large.
When Jesus talks about things that come out of our heart He is talking about the state of our spiritual body. He is talking about the condition of our heart. Some external sensual things certainly are no good for our thoughts and our lives. When we welcome them into our life and befriend with them we are under their spell and the evil intensions emerge. Truly evil intentions dwell within a heart and we are the ones who take all the blame, not a society we live in or our peer group.
God alone can change our hearts and make them clean because God is both the loving God and the author of life and when our spiritual world belongs to God we are in the safe hands.