Personal voice is elusive, and it takes many forms. Essentially, it gives the essay a bit more ‘flavor’ by showing you the author’s attitude towards an issue.
One problem with zoos is that they are created for human entertainment.
Zoos must be rejected for they often seek to satisfy man’s selfish desire for entertainment.
Here, personal voice takes the form of a forceful statement. Note how using adjectives (“selfish”) help to give you more voice.