I can't even recall how many stories I heard about a faithful home teacher who just wouldn't give up. He was assigned to an inactive family who was not only inactive, but seriously involved in un-approved church activities.
In spite of the family repeatedly telling the home teacher that they were no longer interested in the church, that faithful home teacher just wouldn't take no for an answer. In spite of the family telling this home teacher not to come by anymore, that faithful home teacher listened to the spirit and just kept going back...
... and it's a damn good thing he did. Eventually he reactivated this family and now the father is a stake president.
While this story might be true OCCASSIONALLY, when it does happen it is made to appear as if it happens every single time.
This is why they can't take no for an answer. They are spoon-fed with the idea that if you pester your family and friends long enough, eventually they will feel the spirit and join the church. Afterall, their salvation is more important than their request to be left alone. And if you don't pester them, then their sins will be on your head.
So Mormons pester away, completely disregarding peoples' privacy and requests to be left alone. They really think that if someone doesn't get it right away, they eventually will if you pester them long enough!