Here's why:
* "Happy" is already a complete adjective. It doesn't need any additional suffixes to express its meaning.
* Suffixes typically modify the meaning or grammatical function of a word. You might add suffixes like "-ness" to create nouns ("happiness"), "-ly" to create adverbs ("happily"), or "-er" to create comparatives ("happier"), but these aren't directly attached to "happy" itself.
Let me know if you have any other word-related questions!