If you are proud of your country it means that you associate value with everything that is connected to the country. This value is intrinsic (for example everything manufactured in Germany has an intrinsic value because it is of German origin and you are proud of Germany).

The problem is that this line of thought automatically devalues everything not from your own country if it can be compared to something from your own country. Since everything from your country has an intrinsic value to you, everything not from your country lacks this value and is therefore less valuable to you. This would be OK if it was limited to manufactured products but sadly it includes people as well as whole cultures.

So being proud of your country makes you devalue everything from other countries before even considering its real value.