The Last Episode of Dexter Leaves Fans Far From Satisfied

Why is it so many shows end on such a bad note? Remember how terrible the ending of Seinfeld was? How about Cheers? St Elsewhere? And what about one of the absolute worst, and most frustrating, series endings ever, The Sopranos? Not to mention the shows we loved that went downhill, and never returned to their once great standards, like L.A. Law?