PM Laws: Who Defines Project Success?

When joining a project, I often ask team members to tell me a bit about previous projects. It helps me to understand their perspective on project work ... and keeps me from being (as) surprised later.

I'll ask them to tell me about a successful project and what it was that made it a success. I hear lots of reasons such as a great team, got to use new technology, build what 'I' wanted, requirements didn't change, ... When I ask why the sponsor or key stakeholders thought the project was successful, the confusion sets in. Some team members tell me that the sponsor was not happy with their successful project. More commonly, they simply don't know whether stakeholders considered the project a success or not.

This perplexes me. How is it possible for team members not to know what their stakeholders think about the project? This form of 'tone deafness' is a huge risk for project failure.
It's completely fine to be proud of the work we've done on projects, to enjoy being part of a particular team, or to get a chance to work on something really new. In the end, however, the success of any project will be defined and judged by those that initiated and paid for it. Just keeping this on the team radar goes a long way to ensure that the work being done is moving the project towards success.

So, the project you just finished ... was it a success? Says who?