Why did 'Star Trek The Next Generation' end after 7 seasons?

Were ratings dropping substantially? Did they want to end while the show was going well?

The “ending” seems so abrupt. Of course they don’t really stop, but because each episodes’ story is self contained; they couldn’t really build towards some grand finale. The ending was good, but it was just so open ended. Towards the last seasons I felt they really got a good formula down.