Most parents wouldn’t give a loaded handgun to their child, even if promised it would make him or her the happiest kid in the world. Good parents know better than to give their children things that may very well harm them, or others. The same is true of God the Father, who is surely the wisest parent of all!
Yes, Jesus did promise in today’s gospel that the Father will give us anything we ask for, so that "our joy may be complete." And there’s the catch: the Father will grant all our requests, provided they bring us joy. The source of this joy isn’t necessarily those things that bring us pleasure, but those things that bring us closer to him- the source of true and everlasting joy.
"Father knows best," according to an old saying, and this is supremely true of our heavenly Father, who refuses any prayer request that would lead us or others away from him and his joy. That’s why C.S. Lewis could claim that we’ll spend eternity thanking God for prayers he didn’t answer! But only because God wanted to spend eternity with us in the first place.