Simply put, dividends and interest are considered ordinary income for tax purposes. Stocks and bonds are considered for the most part capital assets. You can only apply $3,000 from losses from sales of capital assets (such as stock losses) against ordinary income in any tax year. Any loss above that will roll over to the following years until the losses are fully exhausted.
Note that if you had gains from other capital assets, you could apply the losses to the extent of the gains. For example, if you had a $10K loss on one stock and a 12K gain on another, you could utilize the full 10K loss.
