1 Samuel 14:37

Either,

1. Because he was displeased with Saul for his former neglect, 1 Samuel 14:18,19, where he began to ask advice of God, but was so rude and impious as not to tarry for an answer; and therefore it was but a just retaliation, if when Saul did ask, God would not vouchsafe him an answer. Or,

2. To manifest his dislike of the violation of oaths. For although Jonathan might have a fair excuse from his invincible ignorance and urgent necessity; and though Saul had done foolishly in making this oath (which also God would this way discover); yet when once it was made, God would hereby teach them that it should be observed, and that they should abstain even from all appearance of the breach of it.