It’s your job to bring the positive, not the negative. Meaning it’s not your job to complain, it’s your job to make note of the complaints, then either address or disprove them. Don’t resort to cynicism, because the cynic rarely gets the right thing done.
Not all complaints stem from actual problems. Sometimes they’re a symptom from lack of understanding or skewed focus. Take time to explain why things are the way they are, preferably with a positive spin, and hopefully the disgruntled team member will have a change of opinion – or at least a better understanding, even if you don’t end up in agreement.
It’s a basic human trait to focus on the negative. To obsess over things perceived to require action. We’re also very fast to forget measures already taken, and improvements already made.
It’s not uncommon for very well functioning groups to still be displeased with their “situation”.
So it’s important for you to be able to point to the jobs done, measures taken, and ground gained over time. To validate and evaluate your own work, but also to prove to the team that things are improving.