Great topic. My wife grew up in a High Net Worth family. At one point in her childhoold, her father told her she would never have to work if she chose not to.
They basically cut her off soon after they discovered she was dating me roughly 2 years ago.
No more unlimited credit card paid off by Dad every month, travelling the world and living it up, etc. It's been a rude awakening for her, but it's love, what can you do.
When we first met, we were living in NYC, a ridiculously expensive housing market. Median rent in decent neighborhoods is 3.5K/month, you can make six figures and qualify for tax-abated "affordable" housing. Looking back, I'm not sure how we made it.
We moved in quickly. When money grew tight, at one point I leant her a debit card and told her we needed to save $ X for housing costs, she nearly blew the whole thing without meaning to, she just had no concept that taking a $20 cab ride three times a day was a bad deal when you could buy an unlimited weekly subway pass for $31. I taught her these things, she is not dumb by any means, but budgeting was just never something she'd considered 
When we first met I had next to nothing, and she paid the rent prior to her family ceasing paying her bills. Afterwards, she took undoubtedly humbling jobs when she could find them to help us get by. She demonstrated immense loyalty when I was going through a difficult time, so I have rarely had a second thought about supporting her financially.
She has made enormous progress in learning the value of the dollar, learning to look at price tags and comparison shop, to save receipts and return items, to say no to eating out all the time, to say no to impulse buys (usually!), and that for 99.99% of people, money does not come from an infinitely-renewable magical card.
We relocated from NYC to a far more affordable yet still desirable place to live, to start a family. But money still causes major strife in our marriage. She is our newborn's primary caregiver and probably will not work for a year or two, all the income ($3-4K/month) comes from my end, yet my wife has huge pride and hates feeling dependant. I try to strike a balance between being stingy and not imposing boundaries. It is not easy.
Like some others have posted, she agrees we need to budget, and can sometimes be highly thrifty, but just does not quite understand that 1 hour of work = X amount of money for most people, and that's it, nothing more, period, end of story. That your spending simply cannot continuously add up to more than your income or you become a slave to interest.
I think personal finance skills just become second nature for the vast majority of us, to even subconsciosly consider spending in reference to our costs of living, to make a hundred subconscious calculations when choosing whether or not to make a purchase. But to people who grow up truly wealthy, personal finance can be just a foreign concept.
I manage all the bills and make sure we have enough to survive while planning for a better future. It took me a year to feel like it was a good idea to open a joint bank account. I realized that she had had my amazon account info for 6 months or so and had never ordered anything, so I decided she had changed from that naive girl I'd leant my debit card to a year prior. She is smart. Having a child has taught her how important managing personal finances is.
As far as separation and commingling of finances, we don't have joint credit, just the joint checking account, joint lease. I let her use one credit card of mine, but I think since I have 12 years of credit history but 3 or 4 baddies, she is better off starting from scratch. I am looking into adding Auth User, but even that can add complications, so I'm not too keen.
I watched my parents split even after 30 years, I'd guess they have 1.5 million in assets, and I think prenups make sense.
I wrote up a prenup using legalzoom, stating among other things that if we were to divorce, neither would have claim on any property of the others, other than a joint bank checking account and a primary residence if we ever buy a house to use as savings vehicle. She may inherit, and I wanted it to be clear that I would never go after a dime of that, plus I didn't want either of us owing the other alimony if things didn't work out.
I think the moral of the story is that people can change for the better when it comes to financial habits, but it is usually a time consuming, oft painful process, sometimes with 2 steps forwards and 1 back.
Good luck out there!
I think the reasoning is different for everyone. For me myself it is experience. If I touch a hot burner on the stove with my bare hands I learn fast and won't do it again.
We share a mortgage only because we both needed to be on it to get it. We have a kid together, but that is passive. Whether we are together or not we will both be his parents. We don't look at sharing him like he's an object.
I don't equate love equaling trust, either. Some may say you can't truly love without completely trusting, but I'd say you'd be a fool to think it (just out of experience, that is 
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@gamegrrl wrote:I'm pretty much the money Nazi around here. I don't trust my partner regarding paying bills (we went through that many years ago), so I take care of all money issues and tasks.
Here's the way I see it: Simply put, I am much better at managing money than he is. It's in our best interest for me to handle those things. So I do.
He has a few credit cards on his own now, and I periodically check in with him to make sure he's managing those ok. Today is the day that I ask those questions for the month. I always dread it.
This is how we are, except I do trust my spouse with money. He is actually quite good at it, maybe better than me; he just can't be bothered with it. Given the choice of paying bills/balancing our checkbook or looking at pictures on the "Cats High Five" subreddit forum he would choose the latter. It helps that he is very frugal, and not a spender. When I took over the finances all he wanted were two things: 1) Ensure that a good percentage of our paycheck went to ourselves, whether in savings or retirement and 2) Never, ever, ever carry a balance or pay interest on a credit card.
The only problem is this he is sooo completely removed from the finances he is nearly clueless. An actual conversation we had:
Me: Do you know your Sallie Mae card's login information?
Him: Hmm... Sallie Mae card... is that Capital One?
