It means that "I am too concerned with myself for it to be good for me." Basically, it means that someone is too wary of others and that it has a negative effect on their own life somehow. The statement is often said as a joke though.

The most common meaning of cynical is to mean skeptical of other's people's motivations Thus "too cynical" would mean to always look (too much) for dark reasons/motivations for every thing in life .. which can interfere with enjoying life Oxford New American Dictionary cynical | ˈsɪnək(ə)l | adjective 1 believing that people are motivated by self-interest; distrustful of human sincerity or integrity: her cynical attitude. • doubtful as to whether something will happen or whether it is worthwhile: most residents are cynical about efforts to clean mobsters out of their city. • contemptuous; mocking: he gave a cynical laugh. 2 concerned only with one's own interests and typically disregarding accepted or appropriate standards in order to achieve them: a cynical manipulation of public opinion.