Why does David Talbot lose Merrick Mayfair, who has loved him since childhood, to Louis? Because he holds back. His sense of right and wrong, his old Talamasca discipline is still very much in place, in spite of all his experiences as a vampire. And so he does not give in to Merrick's seduction, for many years rejecting her quite cruelly. Louis, however, does not hold back. Long-suffering and miserable as he has always been throughout the Vampire Chronicles, he does not restrict himself from feeling as David does, he does not overanalyze it as David does, and thus Merrick is irresistibly drawn to Louis's rawness like a moth to a flame. He, unlike David, will give her his all.
David, then, is left lonely, furious, jealous, but it is too late. Merrick is lost to him, forever perhaps.