Keep in mind I randomly thought this up and that there's no study or anything of the sort.
I have this thought that the one thing every single human being has experienced is nature. Even is we've only read about the empowering sycamores or the way a flower has the ability to grow on water. We have as humans not just a vivid imagination, but one that can travel through worlds as we create them. And in those worlds, they all have or take note of lack of it. What happens, no, needs to be in every unknown world? Nature. Think of the movies. They contain different worlds. Think of Wall~E, Pocahontas 2, and The Devil Wears Prada. At some point in each of those movies (typically the climax) the main character has an epiphany, a realization in which they come to terms of missing something. In these three movies, they're all lacking some form of nature they previously nurtured. Plants, which are a type of physical nature the kind you can dream or imagine, the spirit of nature, one of the only or many correlations of physical to spiritual that actually has a connection to one another. And finally, human nature, the link between all three. It's sort of like the word "the" in writing; it connects things together. With human nature we can appreciate both in a way that only makes sense to us. I could say that there's a possibility that a hydrangea feels that way as well, but I'd rather not know. It's sort of like the way two completely different people with totally opposite backgrounds can learn to understand one another, because although it may seem like the two have nothing in common. Of course they do! Physical, spiritual, & human nature. Somehow, between those three, anyone can connect. And you know what? I'd really rather not know hydrangea has been thinking the same thing, because it's against all the natures. Maybe we're only supposed to connect with things on one level, & others two or three. But who knows. This is just my rambling theory. :)