Yes. They should be. It would be a kind of double checking on the relationship quotient. Particularly your love relation. A best friend is usually a relation, who knows everything about you, so far as to be able to judge whether you are going to be allergic to something or not. I mean that as a metaphor but yes, he or she will know immediately if you have been swept off your feet by an infatuation, that will wear out soon. If your best friend does not take too kindly to your lover, it is a sign of problem in one of the two relations. Either he is not your best friend but harboring and hiding his amorous possessiveness from you, or your lover is not the right guy for you. Please take this with a pinch of salt, for it is my personal opinion, and I am not a psychiatrist.