The answer does not lie in their income. Money can make people happy, but it is not the only way. One might be living in a mansion alone and unhappy, while others who struggle for food every day are honestly smiling.
Spiritualists say that the key is not to get attached. It might be true as far as it means that you do not expect certain conditions to be met for the sake of your happiness. However, if it means that you are connected to your friends and family, then it is necessary for your well-being. That is why saying that you should not get attached to anything and ignoring your needs will not lead to happiness.
If a person has been starving for days, you are not likely to smile just because you meditate. You might not suffer because you ignore your needs, but you will not feel satisfied either.
If you have desires – real and reachable ones – then you need to fight for them. Do not expect them to come to you if you are passive all your life. Keep your goals in mind and make a step-by-step plan how to change your life in order to reach it. Do not ignore your desires, but do not make your happiness dependent on them.
That is why you only need to learn one thing: change what you can and accept what you cannot change. If you can make a difference between these two and you can really accept the unalterable, then you will find your peace, and since you change what you can, happiness will come ahead, as well.