Why is low HDL cholesterol bad for you?

Complicated. The conventional wisdom is that HDL helps to remove cholesterol from your arteries and epidemiological studies clearly show that the lower the hdl, the higher the risk and vice versa but drugs that raise the HDL have had disappointing results making the conventional wisdom suspect. It may just be a marker for something else. Bottom line: we're not sure why but it is.
Complicated. Low hdl-c may be related to risk as a marker of other factors associated with cardiovascular disease (cvd) such as high LDL particle number, Insulin resistance, and obesity. Also, low hdl-c may indicate a low HDL particle number (hdl-p). Recent research shows if hdl-c and hdl-p are both low, cvd risk is increased. If hdl-c is low, but hdl-p is high, cvd risk is not increased.