You like it and/or you're good at it. Do you really need another reason? If you need a little more convincing, read on.
Professional graduate schools (business, law, medicine) think it's a great major because they realize that studying mathematics develops analytical skills and the ability to work in a problem solving environment; these are skills and experience which rank high on their list of assets.
Jobs in the private sector abound; careers where a mathematics major is particularly well suited include:
Whether you're interested in developing models and interpreting their results, or are interested in developing efficient algorithms to expedite known processes, mathematics and computer science are the tools of choice.
Models are needed to investigate air flow across the surface
of aircraft wings, chemical and biological processes, astronomical
trajectories and urban development. These models need to be
designed, created, the data from them collected and analyzed,
conclusions drawn and predictions made from them.
Possibly your interest is in the construction of the model;
maybe it's in what the model tells you about the situation being
modelled; maybe it's in how to collect and organize the data
for analysis, or maybe it's in the analysis of the data itself.
Maybe your interest is in developing a system to keep the data
secure, or in developing your talents to circumvent the existing
security of a data system.
In the recent book The Jobs Rated Almanac author Les Krantz ranks 250 jobs according to six criteria: income, stress, physical demands, potential growth, job security and work environment. He obtained his data from the government, trade groups and telephone surveys.
The top ten jobs according to Krantz:
Note that the jobs rated higher than Mathematician also involve significant mathematical reasoning and knowledge.
Here are various links to nonacademic job opportunities for mathematics majors.