Playful lying and dishonesty
For a child, it is hard to accept the boring everyday life. The elders don't mind a boring petty life but for a child everything has to be exciting. So, the boy used the tool of some playful lying to make his story more appealing to his father. If he did not like the wagon, he made it a sled. When he did not like the idea of a reindeer he could turn it to something else. Dr Seuss just wanted children to have fun while enjoying the book. Some fair dishonesty won't hurt anyone. After all nothing in the book is real, it's all the boy's imagination. So the readers may as well just chuckle off.
Storytelling
The details of our everyday life might be boring but storytelling is never boring. It is such a funny thing. In the poem And To Think I Saw It On Mulberry Street, the boy named Marco is a very good storyteller. This may have got into some troubles but he does not care. He just loves to connect with people through exaggeration. He makes everything bigger. His imagination makes the world an amazing place to live in. He even tris other people to experience what he is experiencing. He always creates a great world with his power of imagination. Surprisingly, every time he has appropriate words to describe it to others.
Fiarytale world of childhood
From childhood, kids hear all kind of imaginary stories and fairytales. with time, although they get to know that all of the objects and characters are not real, they still have fancy for it. It is a huge part of a child's life. In Marco's case, what he actually saw was a horse and wagon. Instantly he realized it was a fair opportunity to make an imaginary story of his own. He just went on and on with his imaginary objects and events. There was no room left for reality.
