They didn't fall in love with strangers. Who's a stranger anymore, when your entire life is documented with text, image, and sound? They fell in love with people they knew better, in some ways, than their grandparents had when they married.
That their loved ones were dead to begin with, their digital tracks frozen in time, perhaps made them sweeter, safer. Tragedy is romantic, after all.
As companies realized this, rewriting past lives became another form of real state for ads. And after your death, for a fee, you can have been together since the first day.