The meaning of words can change over the course of time. Words such as "awful", previously used to describe something that filled a person with awe, but now is used to mean the exact opposite.
So it is too with the word "cynic." The word derives from the ancient Greek school of philosophy known as Cynicism. The ancient cynics, however, had a somewhat different outlook on life when compared to the modern day understanding of the word.
Ancient cynics would lead their lives according to the principles of virtue and agreement with nature. They would shun all contemporary ambitions for wealth, power and sex choosing to rather live in poverty.
While the sentiments of the ancient cynics were certainly noble, the modern day meaning of the word is derived from the negative aspects of the original philosophy. Cynics would openly criticise society, particularly for greed and selfishness, both of which they believed brought suffering.
This led them to often be viewed as a group who had a general distrust for the motives and integrity of others. Which is how we understand the meaning of the word today.