I like at it this way. Many people want a sense of community in their life. Community comes from tradition and shared experience. To have "community" in a nation, you need something that transcends ideological differences. Most countries have their shared history often revolving around monarchical attitudes and a tradition based around class, and frankly, they have also had the time to move on and just understand that their country is their country, no matter what form.
The US is young. The only thing we have as a shared tradition in this country that isn't imported is, well, our government. We built it on the minds of other people from different places, but darn it all, that's what 'Mericans wrote that piece of paper. To many, the flag represents those ideals, and since we don't have that long and glorious history to cling to, we cling to our supposed right to life, liberty and the pursuit of happiness.
Honestly, I like a little patriotism now and then. Feels like I'm pledging a personal oath to the ideals written in the Declaration. I also like being overly sentimental towards the flag. It helps remind me of the values we're supposed to associate with democracy and freedom, and personally, I do like to see that flag as one of many symbols I treat with respect. Yet, beyond enjoying the 4th and happily singing the anthem and what have you, I don't like people using patriotism as a tool of division. It should be used to unite people together, not be some partisan means to beginning an "us verses them" campaign.
Too long, didn't read: I actually like a little patriotism now and then, but assholes ruin it by taking it from "idea" to "belief" and turn it into a fucking loyalty litmess test. Fuck that bullshit.