It's different things, for different purposes, but probably at least half the time it's something frivolous. We are saving for vacations, and if anything, my spouse is the one pushing for more expensive hotels, etc, so we seem to be in agreement on that. We only have one income, so there isn't a lot of wiggle room.
Who handles the money management? Is it primarily you?
Whenever my husband started spending too much, it helped a lot to bring him to the computer and "show him the money." I'd say to him, "See this spreadsheet? This is next month's expenses. See that amount on top? When that gets to zero, we can no longer pay our bills in full. We still have 11 days in the credit card billing cycle. That number up there needs to last that long, and I still have to buy groceries and gas and whatever else. Please do not buy anything unnecessary for the next 11 days."
When he saw the actual numbers, it clicked for him every time. We got to a point where all I would have to say is "nothing but needs until (insert date here)."
Every six months or so, I do a rundown of where ALL our money is and how much we have in the various accounts. When he starts talking about vacations, I tell him whether it's feasible or not. I like a good vacation splurge too, so we talk about what we can manage each year. This year, in lieu of one long vacation, we decided to go big and splurge on a couple shorter getaways, one family and one couples only. Next summer we will do a big celebratory family vacation after he retires from the military. As much as I want to join him in the YOLO mentality, one of us has to be the responsible one, financially.
This may or may not be in play for your situation, but oftentimes frivolous spending is a coping mechanism for stress, anxiety/depression, or some other mental health issue. It can be a result of upbringing and a desire to break away from a financially difficult past. Sometimes frivolous spending has much more complex cause than just being blase about money. Maybe think about why he spends frivolously and what purpose does that serve.