it is remarkable how much more expensive it is to live as a single person.
Unquestionably. Consider how many expenses are essentially split in half when living with another person, whether married/common-law/roommate
rent/mortgage, property taxes / condo fees
utilities (power/gas/water) - might use a little more with a 2nd person, but nothing close to double
cable/netflix
car payment/insurance (if you can get by with one vehicle)
Even food expenses are likely to be a little lower - less likely to throw out spoiled food, can buy in bulk, etc.
Now whether giving up complete independence by living with someone else is worth it - probably depends who you're living with!