Sometimes after meditation I lose the joy that I have received from my meditation, and I feel very bad. Why do I lose my joy?
There are two reasons why you lose your joy. One reason is that your mind starts functioning most powerfully and vehemently. While functioning in this way, it allows obscure, impure and undivine thoughts to come in either consciously or unconsciously. When impurity enters, joy has to disappear. But if purity is well-established in the mind, the joy will last for a long time.
Another reason why you lose your joy is that your inner vessel is small, and you have taken light, which is joy itself, beyond your capacity. The quantity of light that you have received during your meditation has satisfied you, but your inner vessel is not large enough to hold it. When you lose it, you feel sad.