A continuous variable changing between 0 and 1 is introduced to characterise contentment, or satisfaction with life, of an individual and an equation governing its evolution is postulated from analysis of several factors likely to affect the contentment. As contentment is strongly affected by material well-being, a similar equation is formulated for wealth of an individual and from these two equations derived an evolution equation for the joint distribution of individuals' wealth and contentment within a society. The equation so obtained is used to compute evolution of this joint distribution in a society with initially low variation of wealth and contentment over a long period time. As illustration of this model capabilities, effects of the wealth tax rate are simulated and it is shown that a higher taxation in the longer run may lead to a wealthier and more content society. It is also shown that lower rates of the wealth tax lead to pronounced stratification of the society in terms of both wealth and contentment and that there is no direct relationship between the average values of these two variables.