Albus Dumbledore was a wise old man. He knew perfectly well if you try to hide something from a teenager, or try to give him half information, that teenager will become more and more curious towards the thing you are making him avoid.
Then why did Albus and the Order didn't make it clear to Harry what Voldemort was after in the start? Including Harry in the order is one thing but keeping him in dark from the things that are ACTUALLY affecting him straight is just moronic.
Why was all that "Special Weapon" non-sense. Something that he didn't have before!! Why not tell him that he is trying to obtain some information that might able him to defeat you and we are trying to protect that very thing from him!
Why did Albus, for an entire year, didn't explain it to Harry the reason he is not looking at him straight. It could have been a 2 min conversation and Albus didn't even had to look Harry in the eye...
Listen Harry, I believe Voldemort is trying to make some connection with you and will try to put false images in your mind so be careful and don't do anything rash! And BTW..the reason I am not looking at you is because he might try to hurt me or the Order by locating /reading the location of our Head Quarter by reading my mind through you.
As simple as that. This could have helped Harry mentally prepare himself and probably taking Occulumency more seriously and above all, Sirius would not have to die because of Harry's rash decision! Why to hide all this from Harry and not tell him up front!! ???
Does simply saying that it was an Old Man's Mistake justify the blunder he did which could have been rectified very easily? It was really good luck that the trio, Neville, Luna and Ginny survived from the death eaters in the Hall of Prophecy. Would Albus have been able to justify himself by just saying it was my mistake to keep you in the dark if any of the 6 teenagers had died too?