Because it's common in Romance languages to use the definitive article before a noun to refer to a generality rather than anything in specific.
"Tiel estas la vivo." = "That's how life is." or "Such is life." Here, "la vivo" refers to life in general, not any life in specific.
"Mi amas la naturo." = "I love nature." Here, "la naturo" refers to nature in general.
"Mi neniam komprenos la virinoj." = "I will never understand women." Here, "la virinoj" refers to women in general.
Mi esperas ke ĉi tio helpu vin!
Yea, I've seen inconsistent use by Duolingo of this. I just came across a sentence "Sxi havas vastan scion pri multaj temoj." Following the logic of "Mi amas la naturon apud la maro," it implies that we should be saying "... la scion" instead of just scion to be consistent.
I don't know any English speaker who would say a thing like this. I get that where we'd normally just say "nature" in Esperanto it's, "the nature", but in this particular sentence the English should also be, "the nature". Or, as others have suggested, another word/phrase entirely.