Vigor: Neuroeconomics of Movement Control

At the airport, waiting outside the security area, you might notice a young family, mom and child, spotting the person they are waiting for: dad. There is a smile on the dad’s face as he exits the security area and finds them. They run toward each other.
At around the same time there is another man, about the same age, also coming out, but not being greeted by his family. Rather, he notices that a limo driver is holding a tablet with his name on it. Not surprisingly, he does not run toward the driver.
Why do we run toward people we love, but merely walk toward others? What brain mechanism compels us to move with more vigor towards things we value more?