Remember King Robert Baratheon? He was a once fierce warrior who won The War of the Usurper and claimed the Iron Throne through sheer strength and force.
But King Robert fell victim to something common to many people (even in the working world) – complacency.
After getting a cushy job ruling the kingdom, Robert let himself go. He began eating and drinking like crazy, paying more attention to feasts and women (other than his wife) than his job as ruler of the Seven Kingdoms. Although he was a decent man with good intentions, he lost focus, took his eyes off the prize, and never tried to improve himself.
As a result, his employees plotted against him, the kingdom devolved into chaos, and his evil son Joffrey (more on him later) began a short but horrible reign of terror. All because he thought he didn't need to keep working on his skills.