I don’t imagine these things work as well in the short term; I’ve also been pretty sceptical about the idea that smiling makes you happier. However, changing your perspective when there really is something you’re failing to notice can be important. Doing stuff for others without receiving any gratitude in return can create a negative feedback loop where you do less for them and they resent the loss of things they took for granted, while gratefulness both shows your partner that you notice the things they do and makes you more likely to actually do things for them in return.
I would actually be less convinced that happier people would necessarily show more gratitude - some people are too self-absorbed to notice that they should be happier (i.e. more content, not merely smiling all the time), or that they should acknowledge an important source of the happiness they have. Giving people like that more things to be grateful for would change nothing.
