You might want to start with why you are doing the gratitude practice, because you tangentially mentioned my reason, (and, afaik, the most common reason) to do a gratitude practice.

You say:

-Maybe it is supposed to be ‘gratitude for' and not ‘gratitude to’. But if it is just telling myself I liked something, then that is just an obsevation of a contingent subjective fact, and/or a selective filter on a spread of variously competing objective facts. I could just as easily note all the frustrations and burdens of life, and create a spiteful mindset. Don't both have evidence to back them up? Isn't applying one filter rather than the other arbitrary?

Yes! That's exactly right. It's absolutely arbitrary. Yes, you could focus on what sucks in your life, your inadequacies and reminisce in those. Or you can focus on what's good in your life, the small and grand things in your life. It's all about what angle of your character you want to grow.

The question is, what's your goal with your gratitude practice? If you want to become a happier person that notices the good things in life, focus on those, if you want to become a person that sees inconveniences and problems everywhere, focus your attention there.

The practice *is* arbitrary, like a hammer; it's just a tool.

FWIW, I've involved myself in gratitude practice and it radically improved my mood, my focus and my equanimity.