People like to believe they are in complete control of their thoughts and feelings, and the idea of hormones can in some way undermine that. The truth though, is that hormones have major effects on almost everything that humans do and feel. This leads to the questions of how hormones affect human behavior.
The first step in answering that question is understanding what a hormone is. A hormone is any chemical created by a cell or organ of the body which gives messages to some other cell or organ in the body¹. In order to receive the message these hormones will combine with a hormone receptor which then sends a signal to the body. The exact nature of how the hormone and the receptor interact varies greatly as do the signals which are sent. Some receptors may react to multiple different hormones by sending the same signal or it even may send several different signals from the same hormone. Many of these are simply used to regulate human processes like eating, sleeping and other basic functions. These can also control behaviors often thought of as more voluntary as well.
One of the best ways to understand just how strongly these hormones can affect someone is by considering conditions in which hormone levels are far different from normal. One of the most wide range of these conditions is clinical depression. In many people chronic depression is caused by hormone imbalances leading to pain, suffering and in some cases suicide². On the other side of the spectrum the drug ecstasy makes use of the hormone serotonin to create the feelings associated with the drug.
Perhaps the most influential of hormonal reactions for many people is that of love. Immediate sexual attraction is generally a hormonal response. What is important about this is that these hormones will react for a period of about one and a half to five years. That time period is also when many seemingly successful relationships end³.
On a wider level many hormones are affected by season. In the darker parts of the year, for example, people are more likely to become depressed and people are more likely to fall in love in the spring. Rarely can individual cases of either of these be attributed to hormones, but the increase in the overall numbers can be.
Hormones affect nearly everything about how humans think and feel. They can control weight, emotion, health and even lower or increase pain thresholds. In fact there is very little in the human experience that is not in some way affected by hormones and their interactions.