Most likely time. They find it hard making some leisure as they are too focused on working and generating more money. And we can actually see the fruit of their labor, living their lives comfortably and luxuriously. But we all know money is not the key to happiness and peace of mind. And that's what they'll realize when they reached their retirement. That they have spend more on enjoying quality time with their family and loved ones, things that are quite hard already if they do it in their old, dying age.
I agree with that, The problem that rich people may often experience is time because what you said is true, when someone is too busy with their work, they will most likely only spend their time working, with their daily lives maybe just waking up getting ready and working then going home to rest and continuing like that, maybe they have free time even though it's only a holiday, but sometimes that day is still used for work. That's right, money is not the key to true happiness because there are still things that can't be bought with money, namely time. Therefore, no matter how busy we are with our work, we must be able to manage our time by dividing it fairly so that we don't regret it in the end. Because most rich people who regret usually don't get time together with other family members.
Time is one factor that often becomes a problem for rich people because they are too busy accumulating wealth. Another problem that rich people fear most is wealth itself, they are very afraid of losing their wealth because they are used to living in luxury and are used to seeing balances with long rows of "0". Another problem that comes to rich people may be the loss of their privacy due to becoming famous for their wealth. In essence, every human being has their own problems, so even rich people definitely have problems, they don't have problems when they leave this world.