It starts when Legolas is seven.
Nobody sees the assassin coming - not the scouts, not the guards, not Thranduil himself. There is nothing for anyone to do.
But Legolas takes his affection for his father, the desperate protective love, and shoves it at the man with the bow - and the man drops his weapon. "I'm so sorry, Father," he whispers, and drops dead, crushed under the weight of emotion not his own.
Legolas just stares, wide-eyed, with everybody else. He had no more idea that he could do that than Thranduil had, than Thranduil or anyone else still does.
It seems like the kind of thing that would be a single, never-to-be-repeated event. It isn't.
Legolas tries to reach out, to sense other people's emotions, and with practice he becomes good at it. He tries to shove his emotions at other people, and fails every time - he can only do it when he's desperate, it seems.
He listens to one of the guards around his father in Council (annoyance, boredom, restlessness), one of the chambermaids in the halls (curiosity, mild embarrassment), the Elf who gives him a bow and teaches him to shoot it (concern, kindness, is that love?).
Never once does Legolas listen to his father. He doesn't want to know what he'll find out.
On his first patrol, they're attacked. Not Orcs, just spiders, but even so it's terrifying.
Legolas takes his fear, shoves it at the spiders - and finds that in desperation it works again. They scatter, not crushed under the weight of his terror but repelled by it.
This time, they realize.
His patrol leader takes him aside, tells him you have a gift, be careful with it. There is nothing in Caradhil's face but sincerity, and Legolas does not try to read his heart.
He can take a hint, sometimes.
He takes Caradhil's words to heart.
Legolas does not speak of it, but in his mind he refers to it as my gift. He tries not to read people; sometimes when somebody is particularly loud he'll hear it, but he does not listen consciously. Nobody wants to have their privacy invaded.
Be careful with it.
He's glad he never tried to listen to Thranduil.
And then -
And then.
Imladris is everything he hoped for and more. The whole valley seems to glow, with pale golden light and with the magic of the place, a tranquil joy that resonates with Legolas's gift like they were always meant to fit together.
The people there are fascinating. More Noldor, more non-Elves, than he has ever seen at once; all like fingerprints. Legolas tries to keep his gift to himself, but even so, he picks up on a whirlwind of emotion.
A red-haired Dwarf, sturdy like redwood. A voice deeper than the creaking of the oaks.
(and oh, sweet Valar, he's beautiful)
Legolas doesn't need to listen, or even to hear, to know that he's in love with Gimli.