Why do I have to pay tax on my social security income?

Up to 85% of your Social Security benefits can be taxable.  There is no age limit for having to pay taxes on Social Security benefits if you have other sources of income along with the SS benefits.  When you have other income such as earnings from continuing to work, investment income, pensions, etc. up to 85% of your SS can be taxable. 

thanks  where do I find the income levels when ss becomes taxable?