When Paul asked God to remove his thorn, God told him not only that His grace was sufficient. He also said “…for my power is perfected in weakness.” Any time I face a problem or obstacle my first instinct is to try to solve it myself, using my own resources.
Sometimes God tells us no to set boundaries, as in the case of the tree in the garden. Sometimes God’s discipline is the reason, as in Moses’s case. Sometimes God’s no to an individual is in the service of his greater plan, in Jesus’s case (And a big hallelujah for that no!)