It was a spark.
A tiny flutter of something Chell couldn't quite place that ignited within her when she stepped through a portal. Usually it was quickly extinguished by a snarky comment from the ever scrutinizing GLaDOS, but sometimes it wasn't.
Sometimes, when she was flying through the air or gliding across gel slicks, Chell found that it was hard not to throw her head back in a silent cheer of thrill. Sometimes it was hard not to become mesmerized by the detail and complexity of the test chambers. It was nearly impossible to walk out of one of those test chambers that you took one look at and thought There is no solution to this you might as well just toss me to the turrets now, without a smile on her face.
The feeling puzzled her to no end, because she hated Aperture. She hated how it smelled like neurotoxin and was covered in blinding white surfaces that reminded her of a mental hospital (which was where she would likely end up if she ever got out of here). She was haunted by the childlike voices of the turrets and the murals left behind by the last poor soul who had walked these halls before her.
What she didn't realize was that she didn't hate Apertures' purpose. Some small, rebellious, mischievous part of Chell, the part of her that made her the crazy, mute, lunatic she was, loved testing. The things she had done in the testing tracks of Aperture were almost beyond belief, from floating in tunnels of energy to literally walking on sunshine. It was thrilling and fun and energetic and nothing made her feel alive more than nearly dying. Chell would never admit it, but she loved testing. How could she not when science was so much fun?