The biggest problem with naivete is how easily it turns into its opposite. I know any number of hippie types who started out with peace and love and human rights and ended up howling about narcissists, sociopaths and perverts.
On a larger scale, the same thing happened with humanism. The humanist writers such as Bertrand Russell expressed despair about humanity. The human species disappointed them. They believed that people were good, then people did what they did during the Second World War. Whereas we see much less despair in people of say Christian perspective, who assume that human nature is sin and expect people to act in destructive manner.
The same people that championed the 1960s peace and love ideal also embraced the 1990s hysteria about perverts and sociopaths. They went from an imbalance of idealism to an imbalance of fascism. They changed their views completely but remained extreme.
The person who is naive gets screwed over in one way or another. They do not see the wrong of which people are capable, and any number of people take advantage of that. The person starts with good will and ends up with ill will. Either people mistreat the person, or the person sees terrible behavior in people. They start out loving people and end up hating people.
The solution is not to see people as either good or evil. The solution is the rational one: That anything capable of choice can be good or bad. We will see good people and we will see bad people. We will see people good in some ways and bad in others. We will see people being bad at some points in their lives and good at others.
The naive stance fails to see evil and falls for it. The cynical stance fails to see the good and portrays anything of any kind of merit as being yet another, sneakier way to do evil. In one decade people fall for all sorts of nonsense; in another they abuse anything that is beautiful and anything that is good. The same people are doing both of the above. Which is consistent with my previous statement: That people can be good or bad at different times in their lives.
Neither naivete nor misanthropy is the solution. The solution is to see things for what they are. That way one avoids being screwed over and one also avoids missing opportunities. Neither emotional state is the correct one. The correct solution is the rational one.