DeCastro is by far the top guard in the NFL draft and the Cowboys have a need at guard. That makes this pick an easy one. Interior offensive linemen picked this high don't bust very often so it's also a safe pick.
SB Nation's Cowboys blog, Blogging The Boys, explains the pick:
The Cowboys interior offensive line was a problem last season, and with the release of Kyle Kosier at guard last week, the position is truly in flux. Dallas did pick up a couple of veteran guards in free agency, but the future of the position is still in question. Enter David DeCastro, who many scouts have said is the best guard prospect to come along since Steve Hutchinson. He's got every skill you need and is a fierce run-blocker. Blessed with agile feet, he's extremely good on the move, and in the Cowboys run-blocking scheme the guards are often asked to pull. DeCastro is projected as the kind of player you pencil into the starting lineup for the next decade. Dallas picked up their tackle of the future last year with Tyron Smith, they could do the same at guard this year with David DeCastro.
David DeCastro 2012 NFL Draft Player Profile (via sbnnfl)