Parents bring up their child with love and care because children are either underdeveloped or immature. But, once a child is developed and mature, a parent has to leave that child. And, also a child has to leave the parent. Except in a few cases, this is the law of nature or a tradition of any culture.
In the similar manner, a student approaches a teacher for knowledge. Teacher makes the student knowledgeable and mature in understanding. When that happens, teacher and student will separate.
All the separations are hard, but they are inevitable. Sometimes they are forgotten due to the future relationships. But, a true love is never forgotten. Parents and teachers love to know the welfare of their beloved students.