"Because you loved her too." Vincent knows that he has to be careful with Elliott -- when Vincent mentions the baby, Elliott asks Vincent if the baby is Vincent's, which Vincent deflects by saying that "the child is HERS, Elliott." As in, what does it matter who the father is? Do it for Catherine's baby, and for Catherine's memory. Vincent also refers to Catherine as "a friend" and not his lost lover. He knows he needs to appeal to Elliott's love for Catherine, not remind Elliott that Catherine loved and chose somebody else.
