Sanderson Mansnoozie had a soft spot for the white haired, winter teen. That was evident by how he let the boy play with his dreamsand while he was off sending the children into blissful sleep. By how, after defeating Pitch, he would make sure Jack only had the pleasantness of dreams be his company at night. He would join the boy in impromptu snowball fights, lend an ear and willing open arms for him to fall into, a warm embrace to fend off the haunting loneliness. He became Jack's partner, his confidant in a way different to the other Guardian's.
It didn't matter that Sandy didn't talk in the conventional sense. Their game of charades became their communication, nurturing a close bond. They could sit in a golden cloud and talk for hours as the Wind sent them adrift around the world.
Jack looked up to him as a Guardian. His eyes betrayed the care he felt when he tried to play off as the nonchalant, rebellious teen exterior he wore as armour. He wanted Sandy's approval, craved his company, his sage advice - and his exquisite taste in pranks. The two partners in crime were usually joined by the hip during the meetings, cunningly planning their next attack under the noses of their future victims - as long as Sandy wasn't falling asleep on the spot.
Jack never let himself admit it out loud, but he had a family now, and Sanderson Mansnoozie, the Sandman and Guardian of Dreams had filled in perfectly for the role of the Cool Uncle Jack had always wished for.