My serpentine belt broke. Why would that happen.

well,seems like the dealer would tell you that the belt was bad,they usually find other things wrong?when doing other work.unless you have a pulley locked up.turn the pulleys by hand while the belt is off,see if one is locked up or hard to turn.the only thing for you to do at this point is replace the belt.it's less than $21.00 for the main drive belt made by Dayco with a 3 year warranty on it at the ZONE!