The thing about laughter is that it brings back joyfulness. Dramatic impressions are not easily forgotten, so when someone can make us laugh, we naturally delight in their being able to do so.
Immanuel Kant counted laughter among the "Emotions by which Nature Mechanically Strengthens Health" in chapter 79 of the Anthropology. Kant writes on the therapeutic powers of laughter:
Good-humored (not malicious and bitter) laughing is, however, more highly esteemed and more beneficial...It may be a hired jester...who makes us laugh or a cunning knave among our friends who seems to have nothing mischievous on his mind. Waiting for his moment, he does not laugh with the others, but then with apparent innocence he suddenly makes his crack (like a taut string). The resulting laughter is always an exercise of muscles which are used for digestion. Laughing helps digestion better than the wisdom of the physician."
Medically speaking, laughing is one of several abnormal forms of respiration like sneezing, crying, and yawning. Mechanically it is produced by a series of short expiratory blasts which provoke a clear sound from the vocal chords and cause at the same time other inarticulate but nevertheless characteristic sounds from the vibrating structures of the larynx and pharynx. The face shows a characteristic expression that is essentially involuntary and often beyond control, but is generally interpreted as a friendly gesture. It can only be imitated imperfectly.