To be clear, while gratitude does improve happiness, the thing that I find compelling is that it improves actual tangible outcomes (e.g. goal achievement, relationship health, doctor visits, etc.)

Also, expressing gratitude for the things you have does not preclude recognizing areas that could be improved. I am grateful that I am able to walk around and use my body for exercise and sport without trouble; this doesn't mean that I think my creaky knees and back ache are good things, it just means that I appreciate those good things that I do have.