Sorry, i will refrain from the attitude
Well one reason is that you may not fear what the government knows about you now but in the past and possibly in the future the moral attitude of the gov has been different. You may find that things you've done in the past may be outlawed and you'll find you're now a criminal. Don't forget that at one point being gay was illegal and minorities didn't share the same rights as others. Laws must be broken for society to advance. If the gov had all this technology 100 years ago many of the things we changed for the better such as equal rights would not exist.
Another good reason is if they are allowed to tramp all over one constetutional right what is to stop them from doing the same to another.
One more is people assume that when people want privacy they are trying to hide something criminal. That's just not true. I don't want them listening to my conversations any more than some peeping tom looking in my window. It't my business not theirs.