Unless you are dating someone who is either a widow/widower or never been married, there is an Ex lurking in the background.  And how your date interacts with that Ex is important.

While I am a firm believer in Exs being civil to one another, I also happen to believe that if your date and the Ex are best friends, then there is still an emotional connection between them.  And, that emotional connection is going to be detrimental to any future relationship that the two of you might hope to have.