Here's why:
* Adverbs of manner describe how an action is performed.
* "Sweetly" tells us *how* something is done, often relating to a quality of gentleness or pleasantness.
Example:
* She sweetly smiled at him. (The adverb "sweetly" modifies the verb "smiled" and tells us how she smiled.)
Let me know if you have any other questions about adverbs!