The Austin said:
Jesus Christ, it was with you for ONE LEVEL. It was exactly the same as every other cube, except it had a heart painted on it.
It was with you for ONE. LEVEL.
That's the interesting thing about the companion cube. It did absolutely nothing. It was just a lump of plastic, like all of the other cubes you hauled around throughout the game. The difference was that GLADOS tried to reinforce the idea that it was your friend, despite it doing nothing. She then expected you to feel sympathy about "killing" it, and was apparently surprised when you didn't hesitate to do so. I always thought this part of the game was supposed to reinforce GLADOS's complete lack of understanding about human emotion. She doesn't really understand how humans form attachments. She thought just by saying the cube was your friend you would care about it.
Then people went around and turned the companion cube into a stupid meme, killing any narrative value it had.