There are countless job characteristics that contribute to making a person happy or making them unhappy, but all of these characteristics culminate in whether the individual is or feels as though they are in control of their life. Self-employed work seems to be the type of work that I would argue brings the most qualities, and the most of each quality, that have been found to correlate with job satisfaction. The self-employed may feel more secure in their job because they will likely feel as though the existence of their position (job) is dependent on how hard they work, and they won’t lose that position as long as they work hard enough. Another major factor that would commonly be seen in an independent job is autonomy. Those who own their own company, or contract for other companies, may abandon a project if they desire and are not help to the wishes of any management.
Those not self-employed but in such a sector as research and development may also feel this same sort of autonomy but within a managerial system. Work intensity, ability to take time off, income, and fringe benefits may be worse for the self employed, but it is always their option to decrease work load (although it may risk their position or company) or take more time off without a guarantee of termination (like one that may be present in an employee position). Those who are unemployed are generally less happy than the employed despite their income. This is likely because those who are unemployed are at higher risk of not maintaining a vision or goal, especially in the short term. The feeling of working toward something can be one that brings happiness, and making successes along the way can contribute to that happiness.
I have had experience in both self-employment and employment and I much prefer self-employment. I’m currently part of three start-ups, one of which I founded and I’m mainly contracting for the other two by programming for them, and I’ve found the work I do for these companies to be the most satisfying out of any work I’ve done for an employer. The biggest factors contributing to my happiness in the work for the company I founded was intrinsic reward and autonomy while the biggest factors contributing to my stress and unhappiness in my work for my company was work intensity and ability to take time off.