"when it's someone important to me, I most definitely want them to know. Secrecy breeds distrust. When people I care about hide things from me I start thinking I can't trust much of what they say."
Edmundia, the issue here is that there can be major differences between what two people want to keep secret from each other. You apparently believe that there should be virtually no secrets among close family and friends. I believe that there are legitimate reasons why people don't want to tell others things they consider private. I wouldn't even want to know that much about the people I cared most about.
But again, the issue at hand here regarding Aslan telling Lucy that she needn't know whether Eustace will return to Narnia is valid. She was asking about what will happen in the future. There was one more similar conversation between Lucy and Aslan in PC. It happened after Lucy first found and talked to Aslan, she wondered if had she gone to him the first time at the gorge, things would have turned out differently, Aslan told her she can not ever know what could have happened.
It should be noted, however, that while Aslan will not tell others of someone else's story, he does not forbid them to tell their own story to others.
MrBob