Derek is better off by himself than joining that selfish child pack, when Derek was an alpha he did everything in his power to save and protect scott he risked his life and his pack he actually lost his pack because of him and saved his sorry ass from his girlfriend's mother from killing him and the little shit basically spat in his face and told him that Derek is not his alpha. Derek saved his life and kept him alive since S1 to S3 and scott never once even thanked him or appreciate what he did for him. so no Derek should never ever join his pack he's way too good for scott to treat him like shit yet again.