The word "grin" derives from the Old English grinning, to grimace; or to even older Germanic cognates for "mutter," "grunt," "groan" and "howl".
"Grin" in the UK still carries a lingering undertone of that older usage. Indeed, the Concise Oxford leans a little that way, defining "grin" as "show teeth, esp. in amusement or pain or in forced or unrestrained or vacant smile."
But the American Heritage Dictionary defines "grin" as, "to smile broadly, often baring the teeth, as in amusement, glee, embarrassment, or other strong emotion." Clearly, when the word crossed the Atlantic (to the US, at least), it lost its connections to the more negative root words.