I love this song because I think the lyrics are really meaningful. I think that Melora is telling us that the Mayfly does so much in one day and appreciates the one day it gets, with no question. On the other hand, us humans get years upon years to live our lives, and we try to extend our time we have on earth. We get so much time on Earth to do something meaningful but some of us just kind of... sit there our entire lives. We try to prolong our existence by doing the stupidest of things, and the Mayfly just accepts the fact that when it's life is done, it's life is done, with no objections or complaints.
I agree, like the meaningfulness in our lives could be summed up in just one day.
I get what she's saying, but the only reason Mayflies accept it is because they have to. They're made that way. They're pretty much not even aware of death and that they will die.