Disappointed with that example Zeldaboy posted. :( Another repeat of what is already in the game with a thousand ways to get focus. omg..Maxis the base game offers zillions of way for the Sim to stay focused and or inspired or any other attribute..why do they keep repeating game play? why? :s If the game was freaking hard then I can understand why packs and or updates would add yet again another way to get focused and or inspired or whatever but omg, repeat, repeat and repeat gameplay. It's like Maxis has no idea they have a zillion ways for Sims to get happy, stay focused, inspired and all the other things...do they even know what the game is about? It's like they just started and said oh, ya, let's add focus attributes to this game! Um..hello Maxis this is why TS4 is a dull game.. because you keep adding the same gameplay.
OP, to answer your question, chemistry matters. You can love a wonderful person but if there is no chemistry don't be surprised when it's over, and they feel like your brother and or sister or best friend but no fire in a relationship. Since TS4 is limited with trait slots and will be with choices the spectrum would be too broad (your Sim will like most of the Sim world) to make it about likes and dislikes or 'preferences' alone. Chemistry has to come in there somewhere.