Automatically is a bit strong...
Betting lines are set so as to entice an equal (or close to it) amount of betting on both sides of the line. Legal bookmakers make money on the transaction (a cut) and use the losing bettors money to pay-off the winners. A good starting point for setting the line is to look at which team is the home team and favor them by X amount of points depending on the sport. The X can be determined by analyzing years worth of data involving games/matches/etc.
For example, Jeff Sagarin at USAToday has determined that the home field advantage in NFL is currently 2.33 for this season. So a bookmaker might look at two equally matched teams and set the line at +2.5 for the home team.
I should add that this would be a starting point for a line. The quality of the teams, injuries, and even popularity can be a factor in determining the opening betting line. This line can move in the time before the game as well based on betting trends.
