Of course you don't just say that. Recall from OP how mirroring works: you listen, repeat, label/distill. Odds are there is some part of what they said that you'd sincerely be curious to learn more about, that you think you might be able to relate to. (If it was a brief "I'm a ___" response, ask questions about the field to get to that point, etc.) And at some point you spin things positive like "But X part of the job is probably exciting, right?" Which is a segue into what you're passionate about yourself after they answer.
The key is to be genuinely curious and show it in body language. For the first part, approach every conversation as if it was a 100-upvote HN posting "Why X is more complicated than you might think." Right? Don't be fake, just rationally hold back the impulse to be cynical. The second part takes practice, but nodding, mirroring their body language, shoulders back open stance, etc. are a good start, and you can feel good about doing that if you are genuine, which shows. It all comes down to empathy and knowing how to show it.