Excerpt from a John Piper article.......

He gives good things. Only good things. He does not give serpents to children. Therefore, the text itself points away from the conclusion that Ask and you will receive means Ask and you will receive the very thing you ask for when you ask for it in the way you ask for it. It doesn’t say that. And it doesn’t mean that.

If we take the passage as a whole, it says that when we ask and seek and knock—when we pray as needy children looking away from our own resources to our trustworthy heavenly Father—he will hear and he will give us good things. Sometimes just what we asked. Sometimes just when we ask it. Sometimes just the way we desire. And other times he gives us something better, or at a time he knows is better, or in a way he knows is better.

And of course, this tests our faith. Because if we thought that something different were better, we would have asked for it in the first place. But we are not God. We are not infinitely strong, or infinitely righteous, or infinitely good, or infinitely wise, or infinitely loving. And therefore, it is a great mercy to us and to the world that we do not get all we ask.