What causes the frequency of dreams?

Perhaps the abundance of dreams a person experiences stems from their constant preoccupation and contemplation of the matters they dream about, or from their frequent anticipation of a specific event. Al-Hafiz narrated from Al-Hakim Al-Tirmidhi that an abundance of visions was common in previous nations, but it decreased in this nation due to the grandeur of the revelation brought by the Prophet, peace and blessings be upon him, and due to the large number of truth-attesters (siddiqin), those spoken to by angels (muhaddathin), and people of certainty within it.