Too much of anything is harmful. Using a good value/quality on a wrong pretext is bad as well. So nationalism or patriotism is not really a negative thing, but it needs to be controlled to avoid negative feelings and actions.
On the good side, patriotism can inspire people to aim high and have great achievements. For example, it can inspire athletes to perform well, good politicos to make unselfish, righteous decisions, men and women to join the army/navy, and various people to do acts of charity. Patriotism has united people to fight for independence, freedom, prosperity, recovery, and justice.
However, where are the limits? Much of the tension in this world is due to this feeling of pride and close belonging, not necessarily for a particular nation, but for a particular culture. Religion may come into play. Religion, like nationalism, is not a bad aspect of humanity, but using it as an excuse for atrocities is not justifiable. The Holocaust may be seen as an outcome of Hitler's nationalism. One of the problems is that nationalism/patriotism tends to blend into prejudice, including racism. Differences in culture become seen as a reason to separate and create a new territory - this is not always necessary, because diversity is the strength of a nation. Vice versa, certain ethnic groups are oppressed, only because they are minorities; yet, true nationalism means embracing all cultures within the nation. A nation is not just territory, but more importantly, the people within it.
Nationalism should not prevent a nation from aiding other countries in need. Indeed, every country has its own problems; however, it would not want to be seen as selfish - a nationalistic government and population should project a good image, ie. willing to help but also working for progress within. The world works according to the golden rule, so if a good number of authorities/politicians are truly nationalistic - for the people, for the good of the nation - and not corrupt, then we would have less problems today/in the future and we would be making good progress in solving other crises.
At the individual or very small level, many people tend to have so much pride in their country/culture that they think they and their fellowmen are the best. Nothing may be wrong with that. Yet it spurs myths, rude jokes, and stereotypes about foreigners, such as "Illegal immigrants are only Mexicans" and "French people lack proper hygiene."
Complacency can result from too much pride. This can lead to ignorance then vulnerability. Back in history, Japan was isolated - although it thrived within for a rather long time, isolation later still caused its decline (every economy benefits from trade). It is important to be always open to outside perspectives, communication, and suggestions for improvement. It is also necessary to be updated with current world affairs, which cause a ripple effect on every nation. Being focused only on what happens locally can make us blind to not only what occurs around the world, but also the positive and negative possibilities that may occur locally. If we always think that we are superior, then we would not be able to see as well as work on areas for improvement and we may become complacent.
Nevertheless, it may simply be best to say, "I am a citizen of the world," as Socrates (and rather Gandhi) did. If we think of ourselves more as citizens of the world, then we overlook many boundaries and cooperate globally on a much better level.
