Nationalism not overdone is good for a nation. Doesn’t it kinda mean you love your country?
People of a given race or culture tend to relate to each other and group together. There are great differences among the various cultures and races. It’s ridiculous to think individuals can be totally comfortable with other cultures.
It’s an acknowledgement, that the best way to organize the world’s population, which consists of mutually-exclusive cultures, is to separate those different cultures into separate Nations.
Think of your neighborhood. Everyone lives in their own house, where they have their own resources and freedom to live according to the way they want, without infringing on others. The houses, in essence, are nations. You can have neighbors with different cultures, and get along because at least you don’t conflict.
Now. Imagine that instead, these neighbors are forced to live under the same roof, and have to share those resources. You can imagine the inevitable conflicts that will arise. That is multiculturalism.
Quite right. It is one of the important results of there being a "melting pot". In fact, it is so important that it is a primary target of any who would seduce a nation.