In "Fahrenheit 451" why isn't Montag able to accomplish this change by himself? Why does he need a mentor?

Imagine if you have lived your entire life one way, with one set of beliefs about the world.  You have spent your life feeling contented and haven't questioned anyone or anything in regards to your life.  Along with that, you have been produced from a society that discourages any sort of individual thinking at all.  School has trained you, almost since birth, to not think at all, but to simply follow along with the masses.  This all describes Montag.  He does not have the tools to think on his own, because he has never done it.  He has no idea why he is unhappy, and why everyone around him is miserable, he just starts to notice that they are.  But how could he possibly know why?  He thinks it has something to do with books, so he tries, on his own, to latch onto books, thinking that they will give the answers.  But it is hard for him.  He has to read certain passages over and over, trying to figure out what they mean, and it is hard going.  The books states, that Montag "read a page as many as ten times, aloud," trying to glean the meaning from them.  He wonders,

"maybe the books can get us out of the cave.  They might stop us from making the same damn mistakes!"

So, Montag does try to find answers to his unhappiness, and his disillusionment with the world.  But he doesn't know how to; he doesn't know why books are important.  And, when he leaves home and tries to read on the subway, society itself undermines him with its constant commercials and noise.  He can't even think straight.  So, he turns to Faber, who he's had a conversation with before, who he senses knows the answers to his questions.  With Faber's help, Montag finally finds the direction that he is seeking.  Faber was there when books were around, helping people to think and find meaning in their lives, so he can help teach Montag.

I hope that those thoughts help; good luck!