What's at stake when we talk about plagiarism is the concept that my ideas, while I may have developed them in great part due to someone else's ideas, are unique to me, and that the time, effort, and research I put into developing those ideas is valuable.
If, for example, I was working on a research paper and decided not to cite the work of another person--whether it was with ill-intent or not--is an act of stealing that person's work; it's cheating that person out of the credit they deserve for working diligently to develop that idea. By claiming someone else's work as my own, essentially I am devaluing the entire process. What is interesting about plagiarism is that if I chose to plagiarize, and then later were asked whether or not I thought the time and effort I put into my piece was valuable, or worth something, more than likely, I would absolutely say yes. The problem is when I steal the work someone else has done, I am saying that it's not actually worth anything. Therefore, I would be saying that the work I've done is not worth anything, either. If you're a perpetual cheater, or maybe don't care so much about what you're working on, maybe this idea wouldn't bother you--however, not everyone plagiarizes, so the issue is that the group that does is stealing intellecual property from those who don't which lowers the value of everyone's work. Plagiarizing is not a singular event: it doesn't just affect the person doing the cheating; it affects a great number of people.