Why the driver hit and run?

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My friend and I were going to have our dinner after the class yesterday.When we were waiting for going across the road,an accident happened.A pick-up suddenly rushed out and hit a small car whilch was on the right way.The driver didn't stop the pick-up and ran.The small car was hit to go aroud and stoped on the roadside. I don't know why the driver run away.Mabye it's the first time he met that,he scared and didn't know what to do .So what's your opinion about that driver.If you were in that situation first time,what were you going to do?