Humans find elves to be beautiful in an otherworldy way and elves find humans facinating and a never ending source of excitement, and so it is inevitable that romantic trysts would occur. A half elf is the inevitable result of these types of relationships.
Living in two worlds, a half elf learns to be a consumate diplomat. She is able to navigate easily between the world of elves and the world of humans. However, they never really feel at home among either society and so many become adventurers to find a place where they fit in.