Prosocial behavior refers to actions by individuals that help others, often with no immediate benefit to the helper. It includes sharing, co-operation and altruism and is a common part of social life. Example: handing a plate of food to a hungry person on the road, giving your umbrella to an elderly woman sitting unprotected in the rain, or rescuing someone from a building on fire.

There are many reasons to explain why and when people render help to others. These various factors are discussed below.

Empathy-Altruism hypothesis

Batson, Duncan, Ackerman, Buckley, and Birch (1981) offered the empathy-altruism hypothesis, which suggests that at least some prosocial acts are motivated solely by the desire to help someone in need (Batson & Oleson, 1991).

Negative-State Relief model

The knowledge that others are suffering, or more generally, witnessing those in need can be distressing. To decrease this distress in ourselves, we help others. (Cialdini, Baumann, & Kenrick, 1981). In this kind of situation, unhappiness leads to prosocial behavior, and empathy is not a necessary component (Cialdini et al., 1987).

Empathic Joy Hypothesis (Smith, Keating, & Stotland, 1989)

It suggests that helpers enjoy the positive reactions shown by others whom they help. An important implication of this idea is that it is crucial for the person who helps to know that his or her actions had a positive impact on the victim.

Competitive Altruism

One important reason why people help others is that doing so boosts their own status and reputation and, in this way, ultimately brings them large benefits, ones that more than offset the costs of engaging in prosocial actions.

Social Learning Theory

It suggests that observing significant others (parents or authorities) engaging in prosocial behavior, causes people to behave pro-socially. Therefore, pro-social behavior is learned (Bandura, 1977; Bandura & McDonald, 1963; Batson, 1998).

Kin Selection Theory (Cialdini, Brown, Lewis, Luce, & Neuberg, 1997; Pinker, 1998)

From an evolutionary perspective, a key goal for all organisms—including us—is getting our genes into the next generation. In general, we are more likely to help others to whom we are closely related than people to whom we are not related (e.g., Neyer & Lang, 2003).

Social Identity Theory

Group identification leads to favouring and conferring positive distinctiveness on the ingroup when compared to the salient outgroup (Hogg & Abrams, 1988).

Defensive Helping 

Sometimes people help others—especially people who do not belong to their own ingroup—as a means of defusing status threats from these people, i.e., to “put them down” in subtle ways and so reduce their threat to the ingroup’s status.

Biological perspective

Rushton et al., 1986 and Rushton, 2004 state that by adulthood, approximately 50% of the variance in altruism, empathy and social responsibility is due to genes and 50% to non-genetic factors.

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Prosocial behavior refers to actions by individuals that help others for no immediate benefit. There are different theoretical perspectives about the causes as discussed above.

Sources:
Introduction to Psychology by Clifford T. Morgan, Richard A. King, John R. Weisz, John Schopler (Click for eBook)
Social Psychology, 12th Edition by Robert A. Baron, Nyla R. Branscombe, Donn R. Byrne, Gopa Bhardwaj (Click for eBook)