User blog comment:DE-Note/What Determines the Strength of One's Aura?/@comment-24993958-20150529235324

Willpower, determination, stubbornness, desperation…pick a word, they all basically point to how much a person wants to achieve something. You have Jaune who (can be reasonably argued to posses all the traits I mentioned) wanted to be a hunter so badly he gambled his life to cheat his way into a test that by all accounts should have killed him, just to have a CHANCE at becoming a hunter.

Another person that has these traits is Yang, that the show likes to focus on her when she is oozing out aura... and then there is Weiss, the dedicated mage of the group; mages being characters that traditionally require a large mana pool to function…except this show calls it aura…which coincidently…

Show’s that every hunter, even the melee ones, are magic warriors with the passive trait of having their mana bar taped to their life bar, so unless they learn damage mitigating skills (like say Ren’s force fields or Yang's semblance), they are likely to run out of aura incredibly fast when suffering direct hits.

But yes basically my answer is that aura is the manifestation of all the potential years they lost due to the amount of stress they exert on themselves.