What brought Frosty the Snowman to life in the Christmas song?

I have a theory... The hat IS Frosty. The snowman is just a vessel, he'll come to life no matter what snowman you put him on. That hat spent it's entire existence at the centre of birthday parties, always aware but never able to see or speak. When the hat was given a humanoid vessel not already occupied the entity within could finally expand it's mind, it's physical presence and it's will. So, when the hat first came fully to life it yelled out the one thing it had always expected it would need to given its existence up to that point; "HAPPY BIRTHDAY!"...