Why do teenage boys cheat?

They never return the girlfriend's calls, or if they do then they say they are somewhere else and don't want to be disturbed as they are with their 'mates'. They act really offhand when they meet up with their girlfriend, and may fly off the handle at their girlfriend for no apparent reason - except there is a reason but they cannot say it for obvious reasons (they want to keep the secret, but the girlfriend may be close to finding out the truth...) They also keep their phone with them at all times, and always empty their email Inbox before they go out.