a litle word-entry

why do both annoy and joy have oy in'em? Now in my mind "oy" is something yucky or something unplesent like oyoyoy. So I understand why annoy has tho oy-factor, but where is the oy-factor in joy? I thought joy was nice. Words are funny like that. Manslaughter is another one. Add S to laughter and you have something worse. Add a man ( not a woman, cause they don't do bad things like that ) and you have somethin ten times worse. funny funny words.