Complaining in the business world can be a good thing for it can be the door to finding new market needs. If you always complain about something then see if you can create a product to solve this problem.
The more common a problem is the more likely is a product that solves the problem is to succeed. If the majority of people are complaining about something then a product that solves their problem is very likely to succeed (See also how to find new business opportunities).
People usually product the things they dislike on social media. By following the news you can easily find out the general problems people are suffering from.
Even if there a product covering a certain need in the market still there can be a room for improvement. Can this product be improved? Can a better product be introduced? Does the current product fully addresses people's needs?
People who have hobbies or who are passionate about certain things are more likely to find out the problems associated with their hobbies or the things they are passionate about. The more time you spend doing something the more likely you are to spot the market needs related to it.
To find more market needs you need to look at negative events from a different angle. Most negative events usually create new needs.
Long ago people had the need for transportation but horses were the only available method. Henry Ford decided to go something totally innovate but creating cars. The new product he introduced solved an existing problem in a very innovate way (See how to become creative).
Frustrated people are always looking for ways to help them deal with frustration. If so many people are experiencing frustration then there is a need for a product that can help those people deal with their frustration.
By asking people about the products they need you can build an idea about the existing needs in a certain market (See also why a market survey can give wrong results).
When you spend a long time studying competitive products you will come up with a list of their flaws. By creating a product that doesn't have those flaws your chance of success will be much higher.
When a person understands a certain market well his chance of spotting a need in that market will become much higher. Only target the markets you understand else you might fail to see the market needs.