Moses talks about the spleen that is laughter and the gall that is anger, and says that anger is better than laughter; this illustrates his point that the righteous are punished now for their sins so that they will inherit the World to Come.
The concept of laughter connects to the Patriarch Isaac, whose Hebrew name means laughter. Isaac is the Left Column which is the Desire to Receive for the Self Alone, or instant pleasure and immediate gratification. Laughter denotes this concept. When we laugh, there is instantaneous pleasure, indicating a direct connection between the Right Column's domain of true pleasure and the Left Column's domain of the selfish Desire for Pleasure. Direct connections, however, create short-circuits. When the positive pole of a light bulb connects directly to the negative pole, without a filament to resist the current, the bulb bursts.
The Zohar is telling us, through the code word laughter, to avoid selfish behavior. Selfish behavior procures immediate pleasure but long-term pain and suffering. When we resist these desires, we feel we lack something "good" for the moment, but there is long-term and lasting fulfillment in store for us. This is one of the inner meanings behind the concept of receiving punishment for our sins in the present and gaining rewards for good deeds in the World to Come.
On another level of understanding, we are told that the Righteous choose to suffer ailments on the behalf of those in their generation who are steeped in sin as a result of selfish behavior. Here, on the merit of the Righteous, past and present, we heal all forms of disease and illnesses, including those related to the spleen and gall bladder. As we think to share this Light with the world, the Righteous no longer have to endure illness on our behalf. All mankind benefits from this Light, as disease and sickness are eternally abolished.