From the Google Search dictionary (similar definition at oxforddicationaries):
1.believing that people are motivated purely by self-interest; distrustful of human sincerity or integrity.
2.concerned only with one's own interests and typically disregarding accepted standards in order to achieve them.
Isn't it slightly contradictory that one can be, according to these definitions, 'cynical of people because they are cynical'?
Another way of saying it is, 'I distrust the integrity of others' motives [I am cynical] because they are unscrupulously self-interested[they are cynical]'.