Why did Paul Revere alert the colonists when the British were coming?

Paul Revere stopped at houses along the way between Boston and Lexington in order to wake the Minutemen, who were private citizens that had formed a militia.  Paul Revere left Boston because he had been warned by his good friend, Dr. Warren, that the British soldiers were going to Lexington to arrest John Hancock and Samuel Adams.  Lantern signals were hung in the church bell tower of Christ Church to warn whether the British troops were traveling "by land or by sea."  Paul Revere "rowed across the Charles River to Charlestown" and then rode a horse across the countryside in the dead of night.  He stopped at every house along the way in order to alert the Minutemen and get them to gather together just in case of trouble.  They were prepared to fight the British troops if necessary.  He eventually reached Lexington to warn his friends.