We live in a "mash-up" culture. You may already be familiar with mash-ups in music: when two or more songs are blended together to make a new composition. Famous mash-ups like DJ Earworm's "United States of Pop 2009" or Danger Mouse's The Grey Album are innovative works of art that reflect the current attitudes towards art and authorship. This mash-up mentality is so pervasive that it shapes every aspect of our digital selves.
Most of us copy and paste videos, photos, quotes, and more on our Facebook or Myspace profiles: your Facebook account can be seen as a pop-culture mash-up that reflects your personality. It is in this sense of copying and pasting that your digital self can be seen as something constructed. Bits and pieces from around the web are collected and you build an online version of yourself.
However, this cut-and-paste approach to the internet does not translate into your academic life. When it comes time to write a research paper, you can't simply cut and paste, because it defeats the purpose. Your professors are looking for your thoughts. You are demonstrating your understanding of the material, and you are creating your original ideas on a topic.
In essence, when you plagiarize, you are not just cheating the person from whom you've plagiarized; you are also cheating yourself. Copying and pasting keeps your professors from seeing the real you. Look at it this way: when you copy someone else's work, all you are showing is what other people think, not what you think.
Here's another way to look at it: let's say you want to work out and get in shape. Someone else cannot go to the gym for you. You have to do the work to get in physical shape. The same is true for your mind. In order to grow intellectually, you have to do the brain work on your own; someone else cannot grow your mind for you.