Why would Jonathan give loyalty to David instead of Saul?
Jonathan didn’t necessarily give all of his loyalty to David. He fights with his father Saul in battle on most occasions. The only problem he had with Saul was how he treated David. They were friends because they were in battle together as well.
We must also remember that David was married to Michal, Jonathan’s sister. This makes David and Jonathan related by marriage. I don’t know what it was that sparked their friendship, but it was one of the strongest friendships in the Bible.
It was more about friendship than loyalty. If Jonathan had truly given his loyalty to David, he would never be seen with his father Saul throughout the rest of Scripture. He was caught between a rock and a hard place the moment that his father began to despise David and become jealous of him.
God used Jonathan to save David’s life on many occasions while he was still with his father. He had a fine line to walk. His best friend and his father were at odds with one another. It didn’t help that David was anointed king while Saul was still alive. If anything, Jonathan had to split his time between his friendship with David and his loyalty to Saul.