The reason is unknown why The Love Boat ended its nine-season run in 1986. But there would be a few TV movies produced after its cancellation. The Love Boat didn't have a series finale, and it wasn't the right show for one.

Actually, the Love Boat ended it's run in 1986 for the same reason most TV shows end their runs, low ratings.

Didn't the Captain get married on the series final?

Didn't the Captain get married on the series final?
Yes, he did. The bride was played by Marion Ross.

One reason for cancellation- the series killer Ted McKinley;)
Seriously, the series had run it's course-Vicki was just about grown up, so her staying on would make no sense, they had promoted Issac, no more Gopher or Julie, Stubing got married. There really wasn't any reason to continue any more, and the ratings were slipping badly.