Science says that the mental side effects of stimulant drugs, like depression, are caused due to the overload of dopaminergic activity in the brain, but when people experience actual great things the dopaminergic overload is basically the same, but without side effects; on the contrary. Seems like the hormonal overloads aren't the central problem at all.
So why kissing a loved one for the first time doesn't cause depression like using cocaine?
For one, drugs work on a very specific neurotransmitter activating a specific pathway, and they also come in great doses. Whereas experiences are much more complex activating different pathways in different way and deactivating others simultaneously. Another point to take is that due to this large quantity of neurotransmitters being released at once, as in the example of cocaine, this will exhaust the synapses via both down regulation of post-synaptic receptors and also depletion of presynaptic neurotransmitters. Yet, this is an oversimplification of a hugely complex system like brain.
To an extent, they do.
Opponent process theory is a neurological concept that tries to explain a wide range of human perceptual phenomena. The general idea is that after some input stimulus, the body tends to infer an "opponent process" that results perceiving the opposite stimulus. This theory has been used to explain visual afterimages, which cause you to perceive a negative color image after staring at a positive color image for a little while. This theory has also been used to examine emotional states, with experiments suggesting that strong emotions tend to be closely followed by opposite ones. Colloquially, this has been referred to as a "happiness hangover", which describes a depressed emotional state that occurs after a highly positive one, such as after returning from a vacation or after everyone at your party has gone home.
