Let's say there is some activity that your significant other wants to do together (going to the movies/opera/a sports event, or any number of things). You, personally, have a neutral attitude towards this particular activity, i.e., the activity itself doesn't give you any particular pleasure/happiness/utility in and of itself. However, you know that this activity DOES have intrinsic value to your significant other; they would be happy doing it in and of itself. However, you also know that they are not willing to do this activity unless a) you are willing to do it with them, and b) you are also getting pleasure out of it (they wouldn't want to do it if they knew that you were only doing it "for them").
My question is this: in this sort of situation, is it better to lie and say that it makes you happy, so that they will do this activity which gives them happiness, because you want them to be happy, or should you instead tell the truth on the principle that you shouldn't lie, especially not to your significant other?