"Sig" is used when referring to a third person as an grammatical object, eg. "Han lär sig" (he'll learn. "att lära" is a reflexive verb in Swedish). As to the arbitraryness of "den" and "det", or why nouns is seperated into different grammatical genders in some languages ... they just are. Our Indo-European ancestors (except the Slavs) apparently thought it was a neat idea to use multiple articles, for phonological or other reasons.