(and why you always lose)
Bookmakers have an edge against you. This edge, or margin, is what enables them to make money in the long run. It gives them the advantage.
Sports betting is considered by most people as a way of enjoying sports. You bet on your favorite team and hope to win money by pure luck. This is precisely what the bookmakers want – people betting like they were playing the lottery. And even if the bookmakers can be slightly wrong, they have a large margin of error. This is why regular gamblers always lose.