Neurotransmitters are how the cells in your (and your pet’s) brain communicate with each other. Chemicals are released from the end of one nerve cell, travel across a tiny gap, and fit onto a receptor site on the beginning of the next nerve cell. This nerve cell is then activated and releases its own endocannabinoids.
It may help if you consider that mammals produce endocannabinoids similar to how we produce endorphins. So, when you think of an endorphin being a natural physiological reaction, so is the creation of endocannabinoids.