Here's why:
* Original Meaning: "Surly" originally meant "strong" or "bold," often with connotations of independence or stubbornness. It wasn't inherently negative.
* Current Meaning: Today, "surly" primarily means "bad-tempered, unfriendly, and inclined to grumble." It has a negative connotation.
Pejoration is the process by which a word's meaning shifts to become more negative over time.