Economics, the science studying the allocation of limited resources which have alternative utility, is often referred to as the "dismal science" because it deals with the inherent, inescapable limitations and realities of the human condition. Scarecity relative to demand will always exist. Personally I find it quite facinating.

Here is an alternative explanation. Nineteenth century economists and evangelical Christians made common cause in the fight against those who argued that blacks were inferior and thus deserved to be enslaved. It was this position that led Thomas Carlyle to brand economics the "dismal science." He claimed that any science that began with the assumption that blacks were human, and thus entitled to make their own decisions about when to work, and for whom, could only be called dismal. On the other side were classical economists like John Stuart Mill, who believed that slavery was wrong and that all men deserved liberty. Quoted directly from http://www.econlib.org/library/Columns/LevyPeartdismal3.html

The dismal science is another, often derogatory, name for economics devised by the Victorian historian Thomas Carlyle. The term is an inversion of the phrase "gay science", meaning "life-enhancing knowledge". This was a familiar expression at the time, and was later adopted as the title of a book by Nietzsche (see The Gay Science). It is often stated that Carlyle gave economics the nickname 'dismal science' as a response to the writings of Thomas Robert Malthus, who grimly predicted that starvation would result as projected population growth exceeded the rate of increase in the food supply. Carlyle did indeed use the word 'dismal' in relation to Malthus's theory in his essay Chartism (1839) "The controversies on Malthus and the 'Population Principle', 'Preventative Check' and so forth, with which the public ear has been deafened for a long while, are indeed sufficiently mournful. Dreary, stolid, dismal, without hope for this world or the next, is all that of the preventative check and the denial of the preventative check." However the full phase "dismal science" first occurs in Carlyle's 1849 tract entitled Occasional Discourse on the Negro Question, in which he was arguing for the reintroduction of slavery as a means to regulate the labor market in the West Indies. Developing a deliberately paradoxical position, Carlyle argued that slavery was actually morally superior to the market forces of supply and demand promoted by economists, since, in his view, the freeing up of the labor market by the liberation of slaves had actually led to a moral and economic decline in the lives of the former slaves themselves. Carlyle's view was attacked by John Stuart Mill and other liberal economists. http://en.wikipedia.org/wiki/Dismal_Science

Because iy deals with "sad" subject matters such as poverty, scarcity, unemployment, taxes, debt, losses etc.