Some of the fastest dogs with the best boxturns seem to have this problem. There is this balance we have to find between dogs that are too ball driven and dogs that don't even bother to bring it back or drop it before they reach the handler.
Certainly, you want some enthusiasm for the little green spheres - having to shape a retrieve is time consuming and frustrating. However, so is having a dog that is so ball-obsessed they crash the box and take off. The happy medium is somewhere in between the extremes.