Why do i get dizzy when i laugh too hard?

Blood flow issues: If your laughing fits are vigorous enough to cause you to stop breathing (even temporarily), this in turn could cause multiple changes in vessel diameters and cardiac output which eventually could lead to low blood flow to the brain. Thereafter the decreased blood flow produces a feeling of near-fainting or dizziness.

Possible.: Sometimes laughing hard can slow the heart rate down and increase pressure in the chest and abdomen enough that blood return to the heart from the lower part of the body is compromised. This can cause a transient decrease in blood flow to the brain and lead to a sense of loss of consciousness.