I think the primary reason to reject patriotism and nationalism is because they are inherently dishonest propositions. The proposed motivation for these feelings is to express love of own country and people but the true motivation is clearly not a love for one's country or people at all but rather the desire to impose a particular type of harsh and violent authoritarian government on the very people and nation that the so-called nationalist purports to love. The true nature of nationalist/patriotic feeling is a will to power.
Because this motivation must be hidden from the masses the belief in patriotism and nationalism is therefore a modern form of superstition, a modern religious myth, if you will. Wherein the state takes the position of an omniscient and omnipotent god in place of a supernatural being.
The idea of a national identity is just as false as belief in supernatural beings, however. Humans quite simply cannot feel kinship with millions of anonymous individuals, therefore quasi-religious belief in an abstract concept that can be adapted to nearly any bourgeois viewpoint is necessary to make the state function. Somehow people must be convinced to believe the illogical premises of statism and this requires the development of a state mythology. Stories such as the one that George Washington chopped down a tree and refused to lie about it serve the same function as any other religious mythology. It's a belief in a implausible, xenophobic worldview that units people under one perspective. If outsiders reject the belief it only proves to the insiders that they were right to distrust outsiders and this strengthens the group kinship..
This inherent false-hood at the heart of nationalism/statism is what makes the pattern of social organization of Fascism so unstable and necessitates the violence and intimidation that are the hallmarks of the authoritarian state.