It's explained in the book I mentioned. You would also need to educate yourself on personal finance by reading some other books to get a more holistic picture (The Millionaire Next Door is a good place to start).

There's three things you need to understand about personal finance:

1. The first is that shit happens and most people don't plan for it (because they haven't been educated on personal finance). Most resources suggest that you keep an emergency fund of 6-12 months of living expenses saved up for when shit happens. We know that most people do not have this. I'm not even going to bother providing a source because there are so many sources on how Americans don't save.

2. The number one expense for any given household is going to be housing (whether owned or rented).

3. While you can cutback on many things, you cannot cutback on a mortgage. So if there are permanent setbacks to your finances: someone loses their job and gets another one that pays less, someone gets hurt and can't work for a while or ends up on disability, someone has to stay home to take care of an unhealthy relative, etc, then you can be pretty much shit out of luck unless you're house has equity on it and you can sell and downsize (although most people will just take a loan out against their equity lol).

So these are the characters of our stage. If you get a mortgage based on 1 person's income and something happens, there is another person there to step up. This provides a powerful safety net for the family. If you get a mortgage based on both people's incomes, you are putting yourself at greater risk because if something were to happen to either person you would not be able to afford to keep your 15-30 year agreement. And the cost of getting out of that agreement is enormous (as we saw with the recent housing crash).

I encourage you to read more on personal finance. It's incredibly fascinating.