Let's try it! Give me more, if I'm not happy with it I promise to go back to what I had

And on the serious side, with respect to health which for me is directly proportional to my happiness, give me less pain which would in this meme mean "more" and I can guarantee you that I will be happy with "more"

This is extremely subjective. It really depends on what you have. If you are homeless and hungry then having more will almost certainly make you happier since having safety/security and not going hungry will make most anyone unhappy.

Having more (and better) healthcare would make many people happy.

Having a nicer home in a safer location would make many people happy.

Having a newer car that is more reliable instead of prone to break downs would make many people happier.

The post seems to make the assumption that everyone lives a comfortable middle class lifestyle and can afford all of the basic necessities of life. That is not the reality we live in.

To what is one grateful?

True unless you have nothing

Exactly, unless maybe you are living in a cardboard box under an overpass.