Women , what's ur obsession with ur friends boyfriend ?

Idk if it's generally the case , or with my exes . I have been in 2 relationships and both wherein her bestfriends showed too much interest in me to a point in the second relationship where it caused them both to fight ( She didn't blame me because I wasn't the one who initiated much ). I heard that people have a thing for someone who is already committed , something like the forbidden fruit . Is it really true tho ?