You find a circle interesting.

Do you also find ovals interesting? You find lightning interesting, but do you find a few atoms blowing in the wind interesting?

Why are some things more interesting than others?

> You find a circle interesting.

Yes, but finding something interesting is a thing that humans do, that's only relevant here if humans are the study object.

> Do you also find ovals interesting?

Yeah they are quite like a circle, but not exactly. Also why do planets "know" how to move? Why isn't it random?

> do you find a few atoms blowing in the wind interesting?

If have the time and the means to study them and I can learn something through that, then sure?

> Why are some things more interesting than others?

To us humans? Because some things are more accessible than others, some things are more generalisable than others, some things are more useful than others. Humans are curious, they like to know properties about things. Why? I have no explanation grounded in natural-science, (only in economics, behavioral science and religion). But that doesn't matter, because these properties stay true no matter what we do. Humans can't know them all, because we are finite and only have access to limited resources.

It's just that these things exist. And they behave. And they are describable by descriptions that are way simpler then they are large. These are complex systems and they are describable by a single number. That's what makes pi special.