I'm good with my 3.125% mortgage. I'm choosing to invest instead.

Made me look.  A similar question was posed on a forum I was following in 2006, and at that time (over a year before I found YNAB), I wrote this:

The top three reasons I want to have no debt:

1) Security
2) Security
3) Security

Let's examine these reasons in a bit more detail.

First, there's security. There is no such thing as job security in today's world. I am blessed with a job that pays me well. If this job were to go away, I would have some trouble finding a job that pays as much. Having no debt means that I could consider jobs that pay significantly less if my current employer goes out of business or decides they no longer need my department. 

Second, there's security. I hope to retire, although I don't expect to retire early. Having a big pile of savings and no debt is security in retirement. Debt is anti-savings, required cash flow, and in general risk and insecurity. In and of itself, that's a reason to not carry consumer debt.

Finally, there's security. Like many other people, I find that there are aspects of my job that I don't like. I'd really like to have a big enough pile of F-U money that it doesn't matter whether I tell my employer what to do. Having consumer debt is contrary to having F-U money, and also requires me to have a higher income in order to service the debt. Being able to live off much less than I am paid, and having a big F-U fund to tide me over until I find other work, is being secure in the knowledge that even though I may take the BS at work, I don't have to. If I'm willing to pay the price, I can go do something else. This knowledge makes work a lot more tolerable.

Carrying large quantities of consumer debt is incompatible with security, undermines future security, and doesn't go well with security.

That's my story and I'm sticking to it.

 What's happened since then?  I found YNAB when I was experiencing a perfect storm of financial changes.  It exceeded my initial goal of getting me through that storm while avoiding dis-saving.  In 2016, when I was 60 years old, my employer changed working conditions in a way that annoyed me.  I looked, and found that I had enough F-U money.  So I retired.

Investing well was part of what made early retirement work a decade after I thought it would be impossible.  Another big part of it was finding YNAB, and learning to avoid spending that wasn't important to me.  That part enabled me to throw more money at investing and retirement savings than I would have without a budget.

The current YNAB sound track is very heavily focused on the situation of being broke and just needing to get ahead of the paycheck to paycheck cycle; I was blessed to find YNAB at a time when it preached Living on Last Month's Income and clarity of how much money is accumulated for what purpose.  Those principles are still very valuable to me in retirement.

I still have no debt, as defined by most people.  In nYNAB terms, I continually have debt consisting of current cycle credit card charges.  Using a card to spend next month's income this month is a fool's game; but I'm quite happy spending last month's income this month, paying for it next month, and getting a dab of cash back from it the month after next.

I want to be debt free because I hate debt and this particular debt, from my divorce, is a psychological burden because I was debt free before the divorce.

I want to be debt free because... I'm tired of being afraid. Afraid I won't have enough to retire. I come from a poor family. The vast majority of my family will work until the day they die. That isn't inherently bad, but they don't do it because it is a hobby or in a field they love, they have to do it to make end meet. I don't want to become complacent and just go through the motions for the rest of my life.

Since starting YNAB I've been thinking about What I want my money to do for me. There lots of things I want, mostly electronics, but there are some things I need. I need to make memories, I need to experience all the world has to offer. The airfare, hotel, and food money are really the bare minimum I need to experience the world and while I may not have the means to travel this year I do plan to budget for it in the near future.