Three reasons why john might easily have lapsed into spiritual despondency while exiled on the island of patmos

First, he may have been Fasting for a long time. This may have included sleepless nights and desperation, clinging to religious activity and rejecting hunger and thirst. This is something that strengthens religious commitment and develops spirituality like nothing else. It is so intense that few people voluntarily practice, let alone successful.

Second, He may have been driven by a spiritual passion, which could be the spirit of God reaching Him. Maybe his visions were so intense he was driven into isolation, to struggle with his mind until he realized the capability of understanding the vision of the Bible. Does that mean he saw the actual future?

Third, he was alone. He was out in a natural place, all alone. If he was sincere with his religion, He would have been very frustrated with God, trying to find out why he was so alone, unable to get what he needed to feel good, namely, the crowds of human companionship. In this state a person's mind starts to get very otherworldly, like a sensory deprivation tank. You see visions and have emotions that are not related to the physical world. Leaving this natural high is easy, but making use out of it is not, so maybe he was pushing his mind to gain more knowledge.