The short answer is that people enjoy laughing. In a romantic comedy like A Midsummer Night’s Dream we see people of all ages and social classes, falling in love and making fools of themselves. What romantic comedies have in common is that we enter a world where things seem to have been running fairly smoothly, maybe not perfectly but people have been managing. Then, something goes wrong, in Dream Hermia wants to marry a different boy to the one approved of by her father and from that point onwards the world is turned upside down. As the characters begin to behave more and more bizarrely we, the audience, see our own follies being acted out on stage and end up laughing, not at the characters but at ourselves.
Of course, there is another play where a girl wants to marry a lad her parents disapprove of, Romeo and Juliet, and that is a tragedy; the other side of the same coin.
hmmm....that's an interesting question because if you think about it, Shakespeare also used the theme of love as a tragedy (see Romeo and Juliet). Most scholars believe that Shakespeare wrote A Midsummer Night's Dream as a light entertainment to accompany a marriage celebration; and while the identity of the historical couple for whom it was meant has never been conclusively established, there is good textual and background evidence available to support this claim.If A Midsummer Night's Dream can be said to convey a message, it is that the creative imagination is in tune with the supernatural world and is best used to confer the blessings of Nature (writ large) upon mankind and marriage. Marriage, then, and celebration, would be the reason that Shakespeare wrote this play as a comedy celebrating love.