Love is an emotion, a feeling of deep affinity for something or someone else. To "love" another person means to care greatly about that other, to want to spend time with that person because it brings pleasure, to want to protect the other from harm, to be willing to sacrifice in order to support the other.
Part of the human nature is that we are social animals. All humans have basic psychological needs that must be met if we are to live as healthy individuals. Included in those basic needs is a need to feel wanted and recognized as having worth by others - a need to feel loved. Another basic need reflects our need to be able to give care and support to others - the need to give love away. People love because human nature needs love in order to exist.