why is everyone so sad it is an happy ending tho?

i guess its kinda bittersweet since tougo will live for eternity while masato will die shortly

Not only will the MC die and the devil have to live for an eternity without him, but the other couple's devil will have to witness his reincarnated contractor's death and rebirth for an eternity as well -without any of the contractor's past memories there to help them reconnect.