Try and go a day without complaining. It's almost impossible. Whether you verbalize it or keep it internal, complaints worm their way to the surface, poisoning thoughts and attitudes.
I've been trying -- with mixed results -- to complain less. It's difficult to stifle the whiny voice that pipes up at the first sign of discomfort, or the slightest hint of insult. Taking it a step further, when I hear that voice start to chime in, I try to turn whatever it's saying around and find something positive behind it. This mental exercise is taken from this video by Jocko Willink. Watch it. It will get your blood pumping.
If for no other reason, I've prioritized eliminating complaints because complaining is not a constructive enterprise. From my experience, it produces only more negative thoughts and feelings. It further sours outlooks and dispositions. It's also contagious. I've seen it bring down an entire office. It is an ineffective use of time and effort.
If time is the ultimate commodity, and complaining is not a constructive enterprise, then why bother complaining? I have too much to do, and I'm sure you do, too.