I read that joy has to be a prerequisite for prophecy? I would like to know what is the reason for this? Thank you

Maimonides [Shmona Prakim 7] explains that in order for a person to achieve prophecy he needs to purify himself mainly by refining his Midot – attributes and personality characteristics. Those who possess bad Midot can't receive prophecy, this applies to all bad Midos, sadness as haughtiness or anger or laziness etc. Happiness is therefore only one of many conditions to possibly receive prophecy.