That was a really great piece of game design that grew out of a problem picked up in playtesting where some players didn't realize they were meant to keep the cube with them to solve multiple puzzles; they'd leave it behind and then get frustrated on the next puzzle.
So they added a heart to the texture and a few voice lines, and not only was that problem solved but they created one of the most memorable moments in the game by making people care about a cube and then having them get rid of it.
"While it has been a faithful companion, your Companion Cube cannot accompany you through the rest of the test. If it could talk - and the Enrichment Center takes this opportunity to remind you that it cannot - it would tell you to go on without it because it would rather die in a fire than become a burden to you."
So they added a heart to the texture and a few voice lines, and not only was that problem solved but they created one of the most memorable moments in the game by making people care about a cube and then having them get rid of it.
"While it has been a faithful companion, your Companion Cube cannot accompany you through the rest of the test. If it could talk - and the Enrichment Center takes this opportunity to remind you that it cannot - it would tell you to go on without it because it would rather die in a fire than become a burden to you."