According to Osho, emotional regression around a master happens because the master is a mirror: his presence strips your social mask and reflects your actual, immature mind. Confronted with your "original face," anxiety arises and you fall back into childhood defenses and soothing habits. The master does not create the regression - he exposes it - so acknowledging the truth, rather than attacking the mirror, becomes the first step toward real maturity.
A real teacher shows your true self, so your pretend adult act drops and the scared child inside appears.