Layers of excuses.
To summarize them (and the contents of this thread) in a more digestible form:
1. You are a prime candidate for this job to begin with.
2. The EchoEvery time the game thinks you might be on the verge of forgetting this, it issues a reminder: you are strong all on your own; that's why you were chosen. You're just the right kind of person with the right kind of mental and physical constitution born at the right time in the right place with the right experiences for greatness. You have the means, motive, and opportunity to be the hero that Hydaelyn needs.
3. The Blessing of LightAs far as we've been told, sometimes Hydaelyn sees someone with the potential to serve for this world's defense and she's just like, "Yeah, that one. You will have The Power!" and She awakens them to the Echo, some kind of fundamental jailbraeking of the soul. This allows the wielder of the Echo to transcend a few inherent mortal limitations by resonating with the souls of others. We can understand their words and intentions without knowing their language, we can relive their strongest memories alongside them from a more malleable "instant replay" point of view, and our soul itself is capable of transcending the body to be either called to the aetherial sea to speak with Hydaelyn, or to - if the ability is mastered - transcend the flesh entirely and live as an immortal wraith, apparently. Also we can continue to break down the barriers of our souls to merge with other souls, including primal essences. We've yet to find an ethical, moral, and helpful reason to make use of those last few. Seems to be a "Dark Side of the Force" kind of thing so far - just don't.4. The Champion FactorThe Echo is a call to arms, but not everyone who hears it understands or heeds that call. Those who knowingly accept Hydaelyn's mission are often gifted a Crystal of Light, a small shard of Her power which allows them to transcend mortal limits and gain extraordinary power. Such heroes are often remembered as incarnations of the gods, or Zodiac Braves, or Warriors of Light, or something of the like. It varies by time and telling. On rare and desperate occasions, Hydaelyn may even defend one of the crystal bearers by projecting Her power through that crystal and into the physical world, a sort of divine intervention that costs Her dearly.5. The UnknownEcho-blessed, Crystal-blessed bearers of the Light are rare enough, but occasionally Hydaelyn will choose a Champion. These are Her strongest and staunchest defenders, the ones She most trusts. They can interfere with the aetherial realm itself and the draw crystals of Light are drawn to them. This champion even seems capable of re-awakening the crystals when they grow empty and dim. These six-crystal heroes are extraordinarily rare, the truest Warriors of Light. The powers to this tier in combination give such heroes a nigh-unfathomable potential, though they must rise to the occasion of meeting it on their own.There is more to the Warrior of Light than meets the eye. Their true nature is yet unknown. Please look forward to it.
Understanding this hierarchy of power is required to understand some of the finer nuances of the story, such as why Midgardsormr so hates you when you first meet him. He figures you for one of Hydaelyn's basic chosen - some insolent insect that climbed the corpse of the Keeper of the Lake himself demanding Nidhogg stand down with no knowledge of Ishgard's transgressions. So Midgardsormr moves to kill you - flick you off his bones and across the horizon, and Hydaelyn blocks the blow, declaring you Her champion (a status which means Midgardsormr is honor bound to be your ally by way of his covenant with the Mothercrystal). For the first few moons Midgardsormr is stuck with you, he thinks you're a pretender leaning on the Blessing of Light like a crutch, draining Hydaelyn dry for the sake of raising yourself up as something greater than you are. (Hence why he blocks your access to the crystals for a time, assuming you'll just get ganked without your crutch.) It takes him most of Heavensward to realize just how bad things have gotten and how much you're capable of.