Loneliness is a complex and usually unpleasant emotional response to isolation. Loneliness typically includes anxious feelings about a lack of connection or communication with other beings, both in the present and extending into the future. As such, loneliness can be felt even when surrounded by other people. The causes of loneliness are varied and include social, mental, emotional and physical factors. Research has shown that loneliness is prevalent throughout society, including people in marriages, relationships, families, veterans, and those with successful careers. It has been a long explored theme in the literature of human beings since classical antiquity. Loneliness has also been described as social pain—a psychological mechanism meant to motivate an individual to seek social connections. Loneliness is often defined in terms of one's connectedness to others, or more specifically as "the unpleasant experience that occurs when a person's network of social relations is deficient in some important way".

People can experience loneliness for many reasons, and many life events may cause it, such as a lack of friendship relations during childhood and adolescence, or the physical absence of meaningful people around a person. At the same time, loneliness may be a symptom of another social or psychological problem, such as chronic depression. Many people experience loneliness for the first time when they are left alone as infants. It is also a very common, though normally temporary, consequence of a breakup, divorce, or loss of any important long-term relationship. In these cases, it may stem both from the loss of a specific person and from the withdrawal from social circles caused by the event or the associated sadness.