Because”happiness” for one, differs from person to person. A pedophile is very happy when they get a new play mate. But their logic & wisdom & sense of right & wrong are completely lost while seeking out that which makes them happy, getting what makes them happy & indulging in what makes them happy.
Also, someone who is committing adultery is “happy” when with the one they are having the affair with. But all sense of duty, loyalty, reasoning, concern for the one/s it may hurt, etc. None of that matters, they just want to fulfill their needs of “happiness.”
But The God says that hell fire is surrounded by desires & Paradise surrounded by hardships.
It is the hardest thing to do, not to fulfill one’s desires, especially, when it is easy to get. But one governs their “desires” out of love & fear of The God. Without the love & fear of The God, one will & does whatever they feel without consciousness. Desire can literally blind a person.
It is better to be content with what one has, why? For one, it gives them time to focus on what really matters. Two, it governs the soul & increases God consciousness & concern for other than self. It keeps one from harming others. And it increases hope that The God will provide more & better. Sometimes without a person even asking Him.
How many of us have been broke & then finds money, someone gives them money, or a better job presents itself? etc. He provides & fulfills our wants & needs in ways we do not even expect.
Get closer to The God. For they will be faced with tests & trials. And will need that support system. There are supplications for everything in our lives. Example: Prophet Muhammad taught the prayer “istaqara.” I.e. prayer on decision making. Broken down, it basically is a prayer said “before” making a decision. You are calling on The God, asking Him if this (whatever it is) is good for you. If it is, then bless you in it. But if it is bad for yoi, then remove you from it & it from you.
After calling on Him & knowing that He will make the best decision for you, it is a relief & contentment of the heart. One feels very confident moving forward towards that thing they prayed about. It relieves the stress of worry whether it will take place & protects against despair if it doesn’t take place.