Because they may have a very valid cause for complaining. But the important thing is to listen carefully enough to find the actual cause of the complaint, and not just the symptoms of the complaint. For example, a passenger may have been on a delayed flight. From our point of view it's important to find out what actually caused the delay - because it may have been factors completely out of the airline's control. And if it was something to do with the airline then we obviously have to try and redress that, so that it doesn't happen again and offer an apology if it's due.