Property is a way to deal with scarcity. Two people cannot simultaneously wear a shirt, for example, so one of them "owns" the shirt. Two people cannot both drive a car to different locations at the same time, so one person "owns" the car. Of course, you can lend your shirt or car to someone else, but the final say as to how those two resources are used belongs to the owner.
The product of intellectual labor, using this scarcity condition, cannot be considered someone's property. The only tangential relationship IP has to property is that one person uses force, namely, the government, to prevent others from using "their" idea for economic gain. This scarcity is artificial because absent that threat of force, everyone would be able to use the idea freely. Physical goods are subject to scarcity as a consequence of physical law; a car acn't be in my driveway and your driveway at the same time. (This argument applies directly to patents and copyrights, but also to trademarks in that a word or phrase can't be used to market something for economic gain.)
Besides the moral argument, which I know some people don't care about (it is subjective, of course), there is a utilitarian argument. Some act as if, without IP law, no one would be willing to create new ideas; it's as if the only incentive for creation is economic gain from a government-granted monopoly on an idea. Just take a look at the news to see that IP law is more commonly associated with patent trolls and corporate legal battles than protecting Joe Inventor's new idea.
At the very least, I wish people would stop comparing downloading movies and music from the internet as stealing. If I steal your car, you can't use it. If I "steal" a song from the record label by copying the exact sequence of bytes that represent a song from my buddy's hard drive to my hard drive, we can both still listen to the song. I understand that some people support the song creator's claim to the song as "property," in the form of IP. However, even then, there has to be a middle ground between copying against the wishes of the song's creator, and theft in the eyes of the law.