I believe that a theme of this story is focused squarely on technology.  Milton is searching for his ideal partner, so he teaches his computer (Joe) what to look for.  At the time the story was written, it had to have seemed ridiculous to use a computer to reduce human personality down to a bunch of algorithms.  Now though, people pay money to use online dating websites that do the same thing.  Asimov's prediction came true.  A theme in the story is that technology is powerful and can be adapted to help people in all kinds of things.  A logical machine can be used to help people fulfill an emotional need.  

The story's technology theme is a double edged sword though.  As Joe learns what it is to feel like Milton, he/it begins to be jealous.  Joe arranges for Milton to be discreetly removed, so that Joe can be with Charity.  

      He's gone, and tomorrow is February 14. Valentine's Day. Charity will arrive then with her cool hands and her sweet voice. I will teach her how to operate me and how to care for me. What do looks matter when our personalities will resonate?
     I will say to her, "I am Joe, and you are my true love."

The story serves as a warning against technology as well.  Technology can be incredibly useful, but it can also be very dangerous.  That theme is echoed in various other short stories too.  For example, Benet's "By the Waters of Babylon" and Bradbury's "There Will Come Soft Rains."  Modern film uses the concept as well.  Terminator has built an entire franchise out of showing audiences the duality of powerful technology.