People within a nation are supposed to look at their fellow citizens of an extended family. The need to build as a national culture is promoted with nationalism and becomes a value, and the other people in the nation adapt to the national ideals, assimilating into the nation. This is an incentive for one nation to want to protect the citizens, because they are of the same clan, based on the same identity. This also allows competition between nations, which allows the best ideas in terms of medicine, science, art, and even religion to compete towards perfection.
Nationalism is resurfacing fast in the current climate of fear for terrorism and emerging countries. On a group dynamic, it is a useful tool when a country requires its citizen to make sacrifices or accept a bad situation, such as an economic crisis (by blaming others) or participating wars etc. It is useful for crowd control and for some people, essential for a strong social identity. It may be good for psychological health as it makes the world more B/W, avoiding many cognitive conflicts that may otherwise depress the person.
The question is asked in a poor manner so I don't know if yes or no is against nationalism. But nationalism is horrendous. It cannot be seperated from racism. It garners support through a false sense of national pride and promotes exclusionary and isolationist policies. It is essentially a reactionary policy masquerading as progressive. Nationalism never thrives in good times, only in bad times where it takes advantage of peoples anger and resentment.
The Scottish Nationalist Party (SNP) are by far the worst culprits of this.
In some cases it would be very important to have people being dedicated to their country, and for obvious reasons, but it usually has negative consequences. It can lead to ethnocentrism, and people might believe themselves to be superior to others. In addition, they might ignore the negative parts of their own country, seeing it through rose colored glasses, basically.
I just think that you are more likely to improve your own country if you view it negatively, but that is just my opinion.