Under carefully controlled experimental conditions, God will behave however he likes.
That's true enough, but God has also given us some examples of how He likes to behave. I'm thinking in particular of Mark 6:
He departed from there and came to his native place, accompanied by his disciples. When the sabbath came he began to teach in the synagogue, and many who heard him were astonished. They said, “Where did this man get all this? What kind of wisdom has been given him? What mighty deeds are wrought by his hands! Is he not the carpenter, the son of Mary, and the brother of James and Joses and Judas and Simon? And are not his sisters here with us?”
And they took offense at him.
Jesus said to them, “A prophet is not without honor except in his native place and among his own kin and in his own house.”
So he was not able to perform any mighty deed there, apart from curing a few sick people by laying his hands on them. He was amazed at their lack of faith.
Not for want of power, but because he would not work miracles in favour
of obstinate and incredulous people, who were unworthy of such favours.
What God wants to do is work miracles in favor of those who have faith in Him.
Combine that with Mark Shea's observation that God will behave however He likes, and you get:
God works miracles in favor of those who have faith in Him.
That's not merely the conclusion of an abstract syllogism. It's an empirical observation of the world we live in. God works miracles in favor of those who have faith in Him. I'd guess not a day goes by that God doesn't work miracles.
How we regard the fact that God performs miracles in our world today -- assuming we ever think about it at all -- has a lot to do with whether any of His miracles are performed in our own lives or at our request.
Do we think of God's miracles as something that could happen, but doesn't? Something that might happen, but won't? Something that does happen, but only rarely, and not to us? Those are good ways of thinking if the goal is for God to not work miracles in our favor.
If we do want God to work miracles in our favor, it seems to really help to need miracles. There's nothing like sure and certain knowledge of your own inadequacy to meet your needs on your own to create room for that faith in God that He seems to love to reward. The people who begged Jesus that they might touch only the tassel on his cloak needed to be healed more than they needed to see themselves as above begging. The people of Nazareth, on the other hand, needed to put Jesus in His place more than they needed healing. Both groups got what they thought they needed most.
These days, we might run the risk of needed to understand the theory and science of miracles more than we need miracles. This gets at the "carefully controlled experimental conditions" part of Mark Shea's Harvard Law of Divine Behavior. There's no way to make God work a miracle, no guaranteed process. Even when God does grant a miracle to someone, there's no guarantee He will grant them the next miracle they ask for. I know a priest with a gift of healing; he says he has witnessed lots of miracles, but he has also prayed over a lot of people who were not physically healed.
How do you have faith that God will work a miracle when you know that God doesn't always work miracles? Strictly speaking, you don't. We have faith in God, in His Son Jesus Christ. We believe -- and at least some of us have had rather striking experiences to back up our belief -- that God is our loving Father, who does not give us a stone when we ask for bread.
Do we know what we need? Do we know that what we ask God for really is bread, and not a stone that we perceive to be bread?
Let me suggest that, if we have the luxury to ask ourselves whether we really need this miracle, then we don't really need this miracle. If my child is ill, I don't stop to think, "Well, perhaps his illness will work out for the good." I do all I can to heal him, and that includes asking God for healing.
Now, all truly Christian supplication includes at least an implicit, "Thy will be done." But this isn't the diffidence of a creature groveling before the Supreme Being, a passive, "But if You don't, never mind, forget I ask," much less a "get out of Your promises free" card we issue in preparation for disappointment. It is rather the recognition of a child that his Father may actually have a better understanding of what the child lacks, and how to provide it.
And even this recognition may well require a degree of theological sophistication the supplicant is too needy to afford. My guess is that a lot of sophisticated requests for miracles go unanswered, because what the requester needs even more than a miracle is to shuck the sophistication and throw himself upon God with the totality of a child clinging to his father.