Instructions
Open the cheat command console by holding down the "Shift," "Ctrl" and "C" keys on your keyboard.
Type in the cheat code "testingcheatsenabled true" without quotations and press "Enter." This will enable the testing cheats, which allow you to manipulate various game functions, including relationship levels.
Click on the Sim you want your Sim to fall in love with and select the "Friendly Introduction" social option. This allows your Sim to meet the other Sim if they have not done so already.
Open your Sim's relationship menu and click on the relationship meter for the Sim you want to fall in love with. Drag the relationship bar over so that it is completely full. This will make the relationship between the two Sims strong enough to move on to the next step.
Click on the Sim you want to fall in love with and select any romantic interactions. Because the relationship level is now as high as it can be, romantic interactions are guaranteed to work. To take the romantic relationship to the next level choose options like "Propose" and "Get Married" from the romantic interactions menu.