The poem starts with the well-known words "Once upon a time", suggesting that what the speaker is going to say is a fairy tale, something so far-fetched it might not even be believed. This makes us think that honesty in expressing emotion is so rare nowadays that it practically is a fairy tale.
The poet creates a contrast between "hearts" and "faces". "Hearts" suggests deep, honest emotion. Thus, when people laughed or shook hands "with their hearts", their emotions came from within. Now, however, they laugh "with their teeth", not with their eyes. It is a cliché that the eyes are the windows of the soul, but they do let us see what a person might be really feeling.
If someone laughs with their eyes, we can see their emotions. But teeth, which are hard, white, and expressionless, reveal nothing. And the people's eyes have now become "ice-block-cold", revealing no warmth. People are now dishonest (while shaking hands, they use the free hand to "search my empty pockets") and insincere, saying things they do not mean.
The speaker tells us that he has learnt to deal with this hard, insincere world by becoming just like all the other people; he too hides his real emotions and speaks words he clearly does not mean. He describes his behaviour in an interesting way, saying that he has learnt "to wear many faces / Like dresses" - like dresses, he changes his 'face', taking one off and exchanging it for something more suitable: "homeface / officeface / streetface" and so on.
We can look at these faces as a series of masks or false faces, which show no real emotion. These faces, unlike hearts, are not sincere. But they are not the faces of evil people either. They are, in fact, the 'social' faces that everyone has to put on in order to deal with all the people they are likely to encounter in their lives. Most of us do wear different 'faces' - that is, we do behave differently - depending on whether we are at home or the office or school or a party.
The speaker wants to be as innocently sincere as his young son. He wants to "unlearn all these muting things"; this suggests that he has learnt how to behave in a way which "mutes" or silences his real emotions. He wants to get rid of his false laugh which "shows only my teeth like a snake's bare fangs" - the comparison with the snake's fangs makes the false, mask-like smile seem dangerous. The speaker regrets the loss of his innocence, but hopes his son can teach him.
"Once Upon a Time" is an emotional poem about the story of a grown up man--who once was an innocent child. His adult world has lost the charm of his childhood years. The poet describes how the process of growing up transforms the innocence of childhood. After entering the adult world, the young adults will gradually forget how to "laugh with their hearts." While growing up, the cold world intimidated our main character. He used to sense
People's insincerity and their superficial laughs, because "they only laugh[ed] with their teeth,/while their ice-block-cold eyes/search[ed] behind [his] shadow" It is a vicious circle: once someone has entered the adult world, he will change--then change others. Our character will learn how to say things that he doesn't really mean: "I have also learned to say, "Goodbye,"/when I mean "Good riddance";/to say "Glad to meet you,"/without being glad; and to say "It's been/nice talking to you," after being bored" 2 Like everyone else, our main character was forced to grow up--in order to adapt to the adult world: "I have learned to wear many faces/like dresses--homeface,/officeface, streetface, hostface, cock-/tail face, with all their conforming smiles/like a fixed portrait smile" In this selfish world, our character learned how to adapt; he adapted a little too well. He now can play the adult role without any problem. However, once he became a parent, parenthood seems to have helped him to remember the innocent world of his childhood. Because of his son, he wants to re-learn how to be sincere.
His son holds the key to this old, forgotten world. What a wonderful poem! It presents in such a simple manner, such a complicated subject: the pain of growing up, and the loss of innocence.