Why don't we stop and help?

There is a news item in Times Of India about a girl falling off from the train and losing her limbs. What caught my eye was her comment that no one came to help. She
called her brother-in-law on her cell phone before fainting. And then an elderly man offered her water and informed the police.

Four years back the same thing happened to a student of mine. He was going on his motorbike and had an accident. He lay on the road but no one stopped his/her vehicle and came to his help. Finally, he pulled his mobile and called his cousin. The cousin rushed over to the spot and took him to the hospital. The time that had elapsed between the accident and the hospitalization was two hours. It was July, the sun was beating down mercilessly and the humidity was high.

I know why we don't stop. The police will ultimately harass us. But somehow that explanation seems so lame. Doesn't humanitarian feelings tell us that we should stop and help? Why do we run away from the scene, head averted, convinced it is none of our business?