Here's why:
* Many suffixes create adjectives: -able, -al, -ful, -ic, -ish, -ive, -ous, -y are just a few examples.
* New adjectives are constantly formed: Language is dynamic, and people invent new words and uses for existing words all the time. A word that isn't currently an adjective could become one with the right suffix and context.
However, some suffixes are more common for adjectives than others:
* -able, -al, -ful, -ic, -ive, -ous, -y: These are very frequently used to create adjectives.
* -ment, -tion, -ity: These suffixes generally create nouns, not adjectives.
Example:
You can't add "-ment" to "happy" to make "happiment" because "-ment" makes nouns (e.g., "movement"). But you could add "-ness" (another common adjective suffix) to make "happiness," which is a noun.
Ultimately, the ability to form an adjective depends on the word itself, its context, and the desired meaning.