What I had to do is first make them good friends. Then go on to use a lot of the romantic interactions. It'll take some time, but they will fall in love.
My favorite part of the game is the fact that your sim can now go around town, and also that you do not have to wait for loading to go to a different lot. I like the new choices in CAS, but there are limited choices in hairstyles. I would like to see an expansion pack much like OFB or University.
It was really hard for me. I had to use a cheat code to make the other sim move in, then that sim was able to propose engagement, and finally marriage.