Why do people complain so much?

Thoroughly enjoyed the cruise on board a very fine vessel. Was it perfect? Not quite , but what in life ever is?. I became dismayed at some fellow passengers whose sole purpose for being on the cruise was to outdo others in the minutiae of what was wrong.
Why do people complain so much over things of such little importance? Britannia is a fine ship with excellent amenities , entertainment and dining options.
The food and service in the specialist restaurants such as Sindhu was excellent and the Limelight club proved to be a fine cabaret club dining and entertainment experience. The buffet dining for breakfast and lunch was top notch and the only downside for us was the evening meal which often failed to live up to the level of expectation created by the menu, and to be honest was just ‘ordinary ‘.
Some people complained about lack of sun -beds , but unless you wanted to be crammed around the main pool we never saw any shortage in any other part of the ship.
Our cabin was very well equipped although it could have done with more drawer space.
Getting to grips with the lifts and lack of stairs in the central atrium was a challenge but this has been well documented and I believe P&O are planning work on Britannia to rectify some of these issues.
Loved the P.O. check in procedure for inbound and outbound flights – this is how travel should be.
As for the holiday – whats not to like about the Caribbean?, For us the absolute jewel in the crown was the rain forests of Dominica – not to be missed.