Why must we focus when listening to teachings/parents/teachers?

During the February 2016 Little Bodhi Seeds Dharma class, Ven. Lama Konchok Sonam talked about "Listen to Dharma Teachings, Parents & Good Friends". Lama Sonam discussed the topic with our Bodhi Seeds in details. He also used the "cup" analogy to demonstrate to the class the importance of paying full attention when listening to teachings, our parents and teachers.

Click this video clip to view the demonstration:

1) Not listening at all = Like a cup turning upside down => The cup can't contain any water
2) Listening some but not all = A cup with a hole or holes => Water in the cup spills
3) Listening attentively fully = A cup not turned upside down and without hole => The cup is able to contain all water poured to it initially