Because, more often than not, you are borrowing money to buy a depreciating asset.
The attraction of a no-fee credit card on which you pay off the balance every single month is that it is a short-term, interest-free loan. You are using some one else’s money, for free. You are getting, IOW, the same deal you gave your boyfriend. But the company is not doing it out of love, but because they hope you will slip up and not pay the balance.
Suppose, instead of borrowing against your credit card, you had dipped into your savings. In that scenario, you lose out on the interest you would have earned on the money you loaned him. Instead, you lost out on the interest you would have earned, as well as the interest you had to pay the credit card company.
That doesn’t mean you shouldn’t have done it. It means that your financial life will always go better with a clear-eyed understanding of what you are paying, and what it buys you. Saving up for a purchase means you are letting other people borrow your money until you need it, and they pay you interest. Using a credit card means paying other people interest for letting you use money you don’t have.
You can get away with paying interest and still come out ahead if you buy assets that increase in value - houses, for instance. You can also get away with it if you are willing to pay for the use of something before you have accumulated the assets to pay for it - car loans come to mind. But car loans and such-like are cheaper to obtain (usually) because they are secured debts. If you don’t make your car payments, they can repossess your car. Credit card debt is typically unsecured debt, and thus the credit card companies have to charge more because of all the people who don’t pay their credit card bills and have nothing that the credit card companies can get to offset the loss.
That is why Lemur866 makes such a cogent point. The ideal customer (from the credit card company point of view) is the guy who carries an unsecured balance and pays on it as reliably as he would for a secured debt, because he is paying interest that he doesn’t have to. People who are that organized and reliable could save up their money and pay cash - but instead they pay 18%.
People often get into trouble because they think that credit card debt is free money - and it never, ever is.