I was recently asked this and, coincidentally, I came across an explanation by Moshe Talmon*:

“The ultimate test for any form of therapy is whether or not a person can utilise or sustain the changes achieved in Therapy.
Therefore, the best time to come to therapy is when you, the client, are ready to do something about your life, and if need be, do it on your own. The key to success in therapy is timing. Are you ready for change now?”

It is generally considered to work best when someone seeks therapy (or schools, or piano lessons), of their own free will; it is much harder to engage with anything when someone else thinks it would be a good idea. But there’s another argument too.

My job as a therapist is to build trust and a feeling of respect and safety, so that someone who is sitting on the fence, or maybe only looking over it, will get down on my side. It is only natural that when someone is uncertain or uninformed about the benefits of something that they hesitate or think it is not for them. In effect it would be the same if I was a school or piano teacher.

* Moshe Talmon is author of Single Session Solutions.