It had to be tough on David knowing he was anointed the next king and here was Saul, whom the spirit of God had left. Here was a holy man in the presence of an unholy person, yet he submitted to him and the Bible says he served Saul. God was trying to prepare David. David was out in the field taking care of the sheep and wild beasts came. By killing those wild beasts he knew that when a giant of a beast stood before him he could do it. But he also had to deal with another beast, a human beast named Saul. Throughout his life David was going to have to deal with difficult people sometimes in his own family. God was preparing him for that.