The main reason for not asking for a raise is fear.

A survey conducted by Salary.com showed that 37% of people constantly negotiate their salaries, while 18% never do. 44% of surveyees answered they have never suggested a pay raise during their performance reviews. The main reason for not negotiating salary is fear of being seen as greedy or of being rejected. So the first step is plucking up courage and deciding to ask for a pay raise. If you don't ask, you don't get it. Moreover, your boss should value your confidence and ambition. Employers are used to dealing with salary negotiations, so it's just a game for them. While for you it's a personal issue, your boss may think, "How can I keep all these people satisfied for the minimum amount of pay?". So when someone asks for a raise, they're thinking, "Oh, well, let's handle this. At least, it's just one person asking for money, not all employees". Remember, this is a game they play all the time. Once you're aware of that, you shouldn't worry so much. Always be nice during the process and follow the tips this micro-course offers to get prepped for negotiating the salary you deserve.