Patriotism is great. Especially when you actively feel it. Seeing your country win at a sport, excelling in academia, or even learning about its past achievements.
In fact nationalism in general has many great effects. It helps, for instance, to create a cohesive society. A nationalistic and proud society is inevitably much more successful and efficient than one with a disenfranchised populace. People are much more inclined to work hard and for longer for something they love and feel a sense of belonging/duty to. Here in the west, nationalism has been a great tool in the invention of the nation sate - a country with well-defined borders and a citizenry (as opposed to subjects). For a long part of European history you allegiance was usually given to kings and seigneurs, not to sovereign nation states such as Britain or France. One of the first documented examples of soldiers fighting for a nation was not till the Napoleonic wars of the early 19th Century where the French, having disposed of the King, had to find a new way of inspiring their troops - as a whole the nation state is a very new concept.
As mentioned previously nationalism can also give a person a great sense of belonging and identity. Identity is perhaps one of the most important human needs. It is a primal need and without which a human can see - amongst many things - a decline in mental health. To quote a 2007 study on the importance of belonging 'The need for social belonging is a basic human motivation' and though this study was primarily focused on belonging in the workplace and on the college campus (with focus on race), the factors mentioned apply equally to the nation state as a whole. Nationalism can also be a tool for poorer countries in their battle with brain drain - that is to say when the well-educated go and live abroad to further their careers and opportunities as opposed to working at home and boosting the local economy. Brain drain has multiple effects on a suffering county (according to L'Université Catholique de Louvain); the emigrated don't pay taxes on income (despite the fact that they send money home), where education abroad is subsidised by the home nation that debt is often never repaid, and it can lead to a shortage of manpower in the mother land itself. A country with a highly nationalistic populace might see lower rates of brain drain as the people see an importance in building their own country more than furthering their own career - there are no known documented cases of this trend.
Nationalism can also be a preserver of culture. Home nation ( Scotland, N. Ireland, and Wales) nationalism is the reason that you can still hear native speakers of Scottish Gaelic, Welsh and Irish Gaelic, and to a certain extent Cornish.
But for all its benefits nationalism doeth have its negatives. Nationalism has as much dividing power as it has uniting power, for identity often comes by defining oneself against what one is not. Linda Colley1 wrote a great article on this exact topic, she put it that:
'National identity, "like ethnic or communal identity is contingent and relational" ... Quite simply, we usually decide who we are by reference to who and what we are not.' Linda Colley, Page 311
In this the 'who' and the 'what' are usually seen in the negative not the positive. In her essay she uses the example of British unity in the fight against the 'evil' that was catholic France. Examples of such nationalism are present in modern Britain. Parties like the BNP and UKIP are strongly nationalistic as a response to what they see as a foreign invasion of culture. Reasonable defence of culture can turn very fast into the hatred of another culture for this reason.
Cultural preservation is equally tricky. Yes it's a great tool to conserve a dying language or tradition, but it can often be misplaced. Traditions are invented all the time and tradition and culture we deem to be ancient and true can be both surprisingly new and historically inaccurate. One need only look at today's highly commercialised and irreligious Christmas, or today's orange tinted, sugar-filled and fancy-dress Halloween. There is a list here of supposedly ancient traditions.
The second paragraph of this article mentions the importance of identity and how nationalism may help shape identity. But this ignores the fact that identity can be fluid and highly circumstantial. This fact was again explained by Linda Colley's essay:
'In practice, men and women often had double, triple, or even quadruple loyalties, mentally locating themselves, according to the circumstances, in a village, in a particular landscape, in a region, and in one or even two countries. It was quite possible for an individual to see himself as being, at one and the same time, a citizen of Edinburgh, a Lowlander, a Scot, and a Briton.' Linda Colley, Page 315
One may also define themselves in many more ways than by nationality. When trying to create identity, focus should perhaps be less on the nation state and more on the individual (personality, hobbies, etc...).
Ardent nationalism can also be damaging to the nation state and when applied to politics it often comes with protectionist tendencies. This can lead to a country being isolationist and uncooperative e.g. Trump's new America. These policies are excellent for successful countries at the peak of their power like the USA today or the UK in the 19th century, but often destructive for the societies outside it. For instance, Britain's era of Splendid Isolation during the mid 19th century in which it refrained from interacting with international politics, a policy that arguably contributed to the outbreak of WWI. This is exacerbated today when you consider the biggest threats to humanity. For instance, climate change is an issue that can only be tackled at a super-national level. This applies equally to genetic modification, nuclear weapons and plastic waste, plus lots more - learn more of these threats here.
This is not to say that there is no place for nationalism. It has its place and will likely never disappear. But it is essential to keep it in check. Radical anything is likely to be bad. You culture/identity is important, but perhaps just as important as everyone else's.
1: Journal of British Studies, Vol. 31, No. 4, Britishness and European-ness: Who are the British Anyway? (Oct., 1992), pp. 309-329
Studies History with Politics at the University of Buckingham