Movies like My name is Khan, Karthik calling Karthik, Paa..... I think these days the trend is to make movies related to diseases so as to gain sympathy of people to get a good ranking of movie, as more people will then turn to watch these movies. More Sympathy means More Business. For instance, the movie "My Name Is Khan" has done a good business than "Three Idiots". What do you think?
That's because most people likes the touching story of the movies. Because of the success of many movies from this sad and sympathy category, it leads to a more production of this type of movies.