Instructions
Introduce your Sim to another Sim you would like her to fall in love with. Click on the other Sim and use the Friendly Introduction social interaction.
Press the "Shift," "Ctrl" and "C" keys on your keyboard if you have a Mac or PC. Windows Vista users may also need to hold down the "Window" key in addition to the rest if this combination does not work. The cheat code text box will appear.
Type the cheat code "testingcheatsenabled true" without quotations to turn on the developer's testing cheats. This allows you to adjust the relationship settings between two Sims.
Open your Sim's relationship menu and scroll to the icon of the Sim you want him to fall in love with. Click on the green relationship bar and slide it all the way to the right. This gives your Sim the highest relationship level possible to the other Sim.
Return to the game and click on the Sim again. This time, select a romantic social interaction, such as "Flirt." Continue to use romantic social interactions until your Sims are in love. Since their relationship level is so high, this should only require three to four interactions to go from friend to romantic interest.