Does is work for you: in any activity there is always an element of risk versus reward. You have to work/ use capital to get that extra unit of income, but that means accepting additional risk. There is an opportunity cost. If you have sufficient income to meet need, a risk averse person will be less inclined to accept the extra risk and s marginal utility decreases the higher the income is. In humanistic terms, It's like if I work any harder, I may actually fail altogether eg health wise and so be unable to work/earn hence I wont risk losing all just for that extra £1.