A wild fan-theory I heard recently is that Lennier was Delenn's illegitimate child from an affair in her youth (possibly with the dead guy from "Legacies"). It explains why he'd be raised in a temple from birth, her choosing him to be her protege sight-unseen despite him having essentially no life or work experience, and, in an unfortunately oedipal way, his peculiar crush he could never quite make sense of. And it wouldn't be the first time Delenn not telling someone the whole truth under any circumstances led to disaster.