Why is Keir Starmer so obsessed with denying the free movement of people?

The EU has offered the UK the chance of free movement for people under the age of 30, but Keir Starmer is adamant that this must not happen. He has offered no good reason, just like the parent who says no to a child without offering explanation as to why. Could it be that having the chance to say no without good reason is why he sought power?