A teacher should celebrate the day a student no longer needs the explanation.

The moment someone no longer needs your explanation is easy to feel as a small loss, since your usefulness in that particular way has ended. It is in fact the completion of what you set out to do. Wanting to remain necessary is human and it quietly works against the aim of the whole exercise.

When someone outgrows your help, say so warmly rather than looking for a new dependency. Then step back. There is usually someone else at the beginning who could use the attention.