LSD, or lysergic acid diethylamide has a structure that is very similar to a few neurotransmitters that are naturally produced. The key it is most similar to is a neurotransmitter known as serotonin. Serotonin is used to modulate and signal a variety of things in the brain. Visual processing (or what you see) utilizes a lot of serotonin. Almost all of the senses have some serotonin input as well. Emotional processing (sad, happy, excited, etc.) is also heavily serotonin influenced.
LSD happens to be even better at activating serotonin receptors than serotonin itself