I'm driving because it's costing me less than half of what I'd pay to take the commuter rail, and public transit is beyond unreliable. I commuted via public transit for 12 years, including eight years after I moved to the suburbs. EZ-Ride is $2 per trip (though it's temporarily free according to their web site). An extra $80+ per month is not a small amount on top of ~$300 for a commuter rail pass.

I'd still be taking the train if it wasn't so much cheaper and faster to drive.