1 .Estel met Legolas for the very first time at the age of seven.(ignoring The Hobbit timeline)
2 .And that was a shiny-motherfucker-fairy-tale-are-you-an-angel-fell-from-heaven moment in his future-king-of-men eyes.
3 .Since the elves in Rivendell are normally brunette,Legolas was the first blonde elf Estel had ever seen.
4 .In elves' immortal life,they spent hundreds of years growing into adults,which means even when they were tiny kids,they barely changed in mere ten or twenty years(a thirty-year-old elf baby,ha).So when Legolas once left Rivendell and came back after a year,he was shocked by the massive change of teenage Estel(taller,deeper voice,stronger muscles etc.)Fortunately,he still was-and will forever be-the sincere human boy he knew. And because of the Dúnadan's longer lifespan, Estel stopped aging after he was fully-grown ,and kept the young look he had in LotR movies.
5 .On Estel's eighteenth birthday, he gave his real name to Legolas as a gift . He called him Aragorn ever since then.
6 .Legolas is the one who taught Estel archery.