What is meant by the cost of capital, as the term pertains to common shareholders' equity? We

Question:

What is meant by the "cost of capital", as the term pertains to common shareholders' equity? We can easily determine the cost of debt, which is the stated rate multiplied by one minus the marginal tax rate; and the cost of preferred stock is usually based upon the annual plus the flotation cost per share for a new issue; but why do we also calculate a "cost" for issuing common stock, other than the flotation? As you may know, a company does not have to pay dividends, and some elect not to, period. With no obligation to "repay" the common shareholders, why do we still consider that there is a cost?
Dividend
A dividend is a distribution of a portion of company’s earnings, decided and managed by the company’s board of directors, and paid to the shareholders. Dividends are given on the shares. It is a token reward paid to the shareholders for their...