Several people have made this point, but since there are a number of others here who understand patriotism as loving your nation in an idealistic, religious way, I feel compelled to weigh in. Most Republicans believe that complaining about your nation isn’t patriotic and that “America” must be praised and considered first and foremost among nations. This is nationalism, not patriotism, though for some people these are the same.
Republicans tend strongly toward nationalism while Democrats tend toward patriotism. A mature love for your nation involves appreciating its goodness, acknowledging the evil it harbors and its bad acts in the world, and working to make the nation better for its citizens and the world. That’s my definition of patriotism.
If you read the comments of those who say Republicans are more patriotic, you’ll see that they almost uniformly define patriotism as nationalism.
This difference in opinion on what constitutes love of country is a common undercurrent in US political discourse. I find it interesting that the majority of Republicans serving in Congress have never sacrificed for their nation, for example in the military, law enforcement, the Peace Corps, … and that their lack of service and their votes against veterans benefits contradict their professed love of this nation.
Finally, I find it very telling that the majority of Republicans support the government when it does things they like and is led by someone they like, but they do everything in their power to cripple the government, even threatening to harm the nation, if they don’t like the actions of the government and especially if they don’t like who’s leading it. That is at best a very shallow, conditional sort of patriotism.
As others have commented, this difference between the Democratic and Republican love of their nation is at least partly explained by the maturity of the love, and the lover, with one very child-like and idealistic, and the other very mature and realistic.