[fxthea]

That's probably a good thing, no? You probably feel discontent because your mind is telling you that you should do something about the situation. Either correct the injustice if it is an injustice or invest in yourself more to put yourself in a situation where you can be earning more. If you are a company where the workforce is in high demand and you are not paying your employees enough to stop them from thinking about these situations then you are going to start slowly but surely swapping out top talent for mediocre talent.

[therandomguy]

My take is that everyone should be always aware of their market value (the reasons for which you have stated). Friend's salary (especially in different domains), I'm not so sure.