It's the sense of touch.

What?

Any real city, you walk, you know?

You brush past people.
People bump into you.

In L.A., nobody touches you.
We're always behind this metal and glass.

I think we miss that touch so much
that we crash into each other
just so we can feel something.

- From the screenplay of 'Crash'
Paul Haggis, Bobby Moresco

The plot has reached a very interesting point now. He loses his good friend, no trace of the body thats riddled with bullets. And he loses the first friend he's made in the city, his face robbed of the jaw that gave him his huge smile, shattered by steel crashing through the windshield of his cab.

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p.s. I'm off comments for a while.