The title should’ve been something like “why do people dislike to speak about their salaries?“, but Jakob Nielsen (a usability leader, but sometimes too radical) suggested to use numbers in the titles.
The people avoid showing their pays to others because they are scared of being compared. It’s a quite silly idea, to use the money you earn as a measure of how good you are. But it’s a numerical, and this means: comparable! Of course, most of them forget the “rest” of things to consider: are you happy with your work, do you have free evenings, do you enjoy your work, etc, and the side benefits (discounts or free use of company’s installations). The thing is that we work just for earning money to live, so the first and big point to consider is the amount of your pay… and sometimes we don’t consider the rest.
Barry Schwartz, in his “The Paradox of Choice: Why More is Less” presentation caught my attention with some ideas, specially this one: “(if your basic needs are supplied) if you have a choice between x or making more money, almost certainly x is what you should choose” (around minute 45 of the video). You don’t need more money to be happier, so then the “rest” becomes more important than the salary.
But it’s difficult to avoid this human behavior which forces us to compare with others. For example, I got angry when I discovered that a coworker, who is IMHO a total incompetent, earns quite more money than me. Anyway I suggest you to consider not asking for more money, but for more “rest”. The first time I asked my boss for a rise, I changed my mind and instead, I asked for less work hours (with the same salary)… now I work 35h per week (the normal thing is 40h), and the other coworkers envy me, because I go home earlier. Enjoy life!