He becomes careful in the use of such permissible things that give him doubt that their use might make him avoid certain mandatory things.

For example, it is permissible to remain awake till mid-night in conversation with friends. In that time there can be two or three rounds of eating snacks. But, if one sleeps late on a full stomach, he might find it difficult to stir out of his bed for the Fajr prayer that is mandatory. Therefore, a pious person develops such a faculty in his nature that he takes care to avoid every permissible thing that might cause difficulty in performance of his mandatory duties.