When you are using only your discretionary income to invest in Bitcoin it means you are investing with peace of mind. Even if Bitcoin falls, your life, bills, and responsibilities stay safe and intact, so use only your Discretionary income for Bitcoin investment or accumulation.
I've seen investors who have that YOLO mentality, everything that they have invested on Bitcoin. In our perspective, there is nothing wrong with that ,as we're all Bitcoin enthusiasts. But on his personal view, he has to know if that will be a good positioning and without having to think of any financial trouble if something happens to them. That's why investing with your discretionary income or spare money, makes you move from both worlds of Bitcoin and real life to be responsible without having problematic of bills and any other obligations that you might stumble upon with.
We all know cases of people who sold their house for Bitcoin and lived in a tent, suffering during the bear market but convinced about their decision, to eventually see the price rise again and become rich. But most of us cannot and shouldn't act that way.
When the price starts to fall, and you invested the money you can't afford to lose, it is unavoidable to start thinking what will happen if the price never goes up again. If you invested your discretionary income, it is still a loss, but you can forget about it as if you'd spent it normally.