I do not think that pride is completely bad. Pride is good for one's self-esteem and so is patriotism.
Pride is alright if it improves the own self-perception. But I don't like it as soon as pride makes people look down on others. This is arrogance. Sadly pride and arrogance are close to each other.
The flag is the most important symbol of a country. It expresses that people are proud of their country. Many years the German flag has not been shown very often in public. But this changed since a few years because of football. Football is the most favorite sport in Germany. Almost everyone identifies with the national team and the flag and its colors demonstrate this identification in every country. Many football fans wave the flag, paint it on their face or put it on their car. I do not like it because I think they are too imprudent.
Is their behavior patriotism? I think so. Patriotism is the love to one's country. There are many reasons for patriotism. Some patriotic people have strong nostalgic feelings and cultural values are very important to them. Some other people have social reasons instead of personal reasons.
Especially in football I think that the main function of patriotism is to give the patriotic person the feeling that he belongs to a special kind of social groups: to a nation. He is a member because he lives in the same country or comes from the same country as the other members of this group. They have something in common. So they feel close to each other and can identify with their nation.
Identity is very important for all of us because it is the answer to one of the big questions in life:
Who am I?
The dark side of the coin is that the closer a person feels to the members of a social group