1) When families are close or people know each other, like a friend of your sister and it might be good might not, since it can go either, if it goes sour especially if they are family friends, it might cause a major break in the ties. Offend them like she isn’t good enough etc..
I know a story of two guys best friends that their families were good friends too, redt his sister to his friend who knew her for years, they got engaged major simcha yay!
Then they broke the engagement, families at war w/ e/o no more friends. The two best friends, now don’t speak to each other.
2)If the person is picky it’s a safer bet to let down a stranger.
3)You ever heard the expression “too close for comfort.”
4)People like change, feel better when it’s someone new, unknown.
I don’t know why exactly, it’s just the way it is.