The law states that all life insurance dividend payments must be equitable. The amount paid to each policy owner must be commensurate with their contribution to profits (at least roughly). This is judged based upon the size of each policy. A policy with a higher cash value will pay a higher dividend generally speaking. During the early policy years, there are load charges which reduce the dividend. Generally these decrease every year until they cease at some designated policy age. The load charges are a way to account for the administrative expenses associated with opening the policy.
The dividend payment is a return of surplus profits. Surplus profits are the profits after all mortality expenses and operating expenses are paid, and after reserve funds and contingency accounts are appropriately funded. While the financial controls of life insurance companies are extremely tight (statistically based and very sophisticated), the possibility for certain expenses to higher than expected is possible.
For example, a natural disaster, terrorist attack, or disease could create a much high mortality expense than what was planned by the life insurance company. More mundane but equally possible a life insurance company’s underwriting department could be judging risk incorrectly and the actual rate of death is higher than anticipated. A higher than expected mortality rate would lead to lower than expected profits, and impact the dividend payment negatively. In the event of higher than expected mortality rates the company will also probably need to fund the reserves at higher rates, which further impacts the dividend payment rate.
Surplus profits are also influence by the rate of return of the investments an insurance company owns. If their fixed income portfolio is negatively affected by interest rate fluctuations (either affecting their interest rate of return or the value of the underlying assets), the company will have lower profits and therefore the dividend payments will likely be reduced.
As you can see dividend payments are not guaranteed. That being said, most major life insurance companies in the United States have a long history of making healthy dividend payments to their policy owners. Some have not missed a dividend payment in over 100 years. Dividends are not technically guaranteed, but they do have a high probability of being paid regularly. Ultimately the board of directors decides the rate of dividend payment to owners.