It was a combination of things for me. Working on an interesting product helped immensely but I think the key for me was respect. I respected the work of the developers in creating the product, and they respected my ability to exercise it and (reasonably) document and report issues.
My last job in the corporate world was testing software. I looked at it as a puzzle - I needed to understand the purpose of the tool but I also needed to think about how our customers would use the product. I looked at the process like a puzzle - combining intent, use, and imagination to come up with a series of tests that would exercise the product.
I always had a good working relationship with the developers. They would often give me early releases - as in not yet really ready for testing - for a sanity check. This was a time for discussion and feedback.
I guess I just liked trying to break things!