American Express just had a terrible six months - and it was perfect for investor Warren Buffett

As an American Express shareholder, the last six months have been perfect for Warren Buffett.

In his interview with CNBC Monday morning, Buffett said that he is "not happy" when a stock price increases because it becomes more expensive for him to buy an additional stake in the company.

Alternatively, when the price of a stock Buffett owns falls, he can less expensively increase his stake in a company.

And what makes a falling share price even better is when a company repurchases its own stock, thereby reducing the shares outstanding and increasing Buffett's stake.

And so not only are American Express shares down about 8% in the last six months, but the company has been repurchasing its own stock. This saw Buffett's stake in AmEx increase to 14.8% in 2014 from 14.2% the year before as the company repurchased $4.4 billion of its own shares.

Buffett's ownership of Coca-Cola and IBM also rose through share buybacks in 2014.

So, Buffett was able to get a bigger stake in the company at less of a cost than if Berkshire had bought more shares directly.

Buffett gave an idea of how much Berkshire's ownership of American Express increased, writing in his annual letter:

If you think tenths of a percent aren't important, ponder this math: For the four companies in aggregate, each increase of one-tenth of a percent in our ownership raises Berkshire's portion of their annual earnings by $50 million.

Companies buy their own shares if they believe the price is undervalued, reducing the number of outstanding shares and potentially lifting their price.

What's more is that Buffett is bullish on AmEx (as well as Coke, IBM, and his final "big four" investment: Wells Fargo):

These [big] four investees possess excellent businesses and are run by managers who are both talented and shareholder-oriented. At Berkshire, we much prefer owning a non-controlling but substantial portion of a wonderful company to owning 100% of a so-so business.

Wall Street has disagreed of late on American Express. The stock has tumbled around 12% year-to-date as it lost its Costco co-branding agreement to Citi and Visa, lost a similar agreement with JetBlue, and lost an anti-trust lawsuit.

But then again, Buffett would rather be able to buy the stock at a lower price if he wanted to than have to chase an increasing stock price of a business he liked.

And as he said on CNBC on Monday: always have your own reasons for buying a stock.