Why is there no "the"?

I notice there is never a "the" for nature, why is that? (E.g. Be kind to nature and we will be benefited. )
The universe is unique, and so is the sky and our sun, but they always come with "the." (E.g. We are insignificant when compared to THE universe. Every time we look up to THE sky, we wonder why THE sun is still there.)
Puzzled.

[Tdol]

I'd say it's because nature is a multi-faceted thing and more general than the sky or the sun.

[rewboss]

"Nature" is also abstract, like "beauty" or "hatred". We're not talking here about any particular nature, but nature in general.

Sometimes we can talk about some specific nature. For example, "It is in the nature of humans to explore." Here, we are talking about a specific nature.

"The sun" is an object (a star, in fact), and has a definite existence. We can point to it. If we stare at it for too long, we may go blind. "The universe" also exists in a very tangible way: it contains everything that there is. "The sky" doesn't, in actual fact, really exist as such, but it looks to us as if it does, and we can certainly point to it and say, "Look, there's the sky!" We can't do that with nature, but we can point to a flower and say, "This is an example of nature."

[Tdol]

Abstract- that was the word I was looking for.