I would love to know why you all got end the show after 15.I look forward see you guys every week. supernatural got other ways to go,they don't have to end.I hope you all leave a cliffhanger that way you can return. I think last one should be a 2 hour

They wanted to end the show on their own terms, rather than dragging it on and risking it becoming less popular. They didn't want it ending due to cancellation, particularly with the cliff-hanger endings the seasons usually had. Knowing that season 15 would be the final season enabled them to tie up as many loose ends as they could, bring back loads of earlier actors for cameos and reference previous scenes. As to why they chose the 15th season as the last - they just felt 15 was a good round number to go out on.