There are significant health risks associated with smoking pot, including serious mental health issues for some people. Quoting from http://www.drugabuse.gov/publications/drugfacts/marijuana:

Research clearly demonstrates that marijuana has the potential to cause problems in daily life or make a person's existing problems worse. In fact, heavy marijuana users generally report lower life satisfaction, poorer mental and physical health, relationship problems, and less academic and career success compared to their peers who came from similar backgrounds. For example, marijuana use is associated with a higher likelihood of dropping out from school. Several studies also associate workers' marijuana smoking with increased absences, tardiness, accidents, workers' compensation claims, and job turnover.

I know "correlation doesn't equal causation". But there are clear risks associated with marijuana use that should not be dismissed so casually.