Common sense would tell us that stiff, hardened arteries are not good for us. And common sense would be dead right. The medical term is "arterial stiffness" and one study found that "aortic stiffness expressed as aortic PWV is a strong predictor of future CV events and all-cause mortality."  In other words, if you arteries are hardened, you have a much greater chance of dying, especially from heart-related issues.
There are several reasons for this, including the fact that stiff arteries produce a higher spike in the blood's pulse pressure wave and that wave has more velocity. Those spikes, over the course of years, can damage tissue (including brain and kidney) and lead to additional arterial plaque buildup just for starters.
Peak Testosterone readers know just how interrelated the heart and penis are and will not be surprised to find that arterial stiffness is also related to erectile dysfunction.  We all know that you need nice, expandable arteries for blood flow into the penis, eh?
So what causes arterial stiffness and hardened arteries? Below are some of the culprits that research has recently uncovered.
NOTE: For practical solutions, see my link on How to Partially Reverse Hardening of the Arteries.
1) Advanced Glycation End Products (AGEs). Young, supple arteries are composed of undamaged elastin and collagen. Part of the aging process in everyone is for the bonds of these proteins to be broken and damaged for AGEs, which stiffens arterial walls like old leather.  However, many people have accelerated AGE damage. Advanced Glycation End Products will damage your body in a hundred other ways as well: read my link on The Dark Side of AGE's for more information.
2) Hypertension/High Blood Pressure. Any extra pressure on the walls of your arteries is "death by a 1000 cuts". The extra pressure slowly damages arterial tissues, contributes to plaque buildup and other nasty phenomenon that lead to increased arterial stiffness.  Again, you want that pressure down below 120/80. See my link on How to Lower Your Blood Pressure Naturally for ideas. (Always discuss changes with your doctor first, especially if you have a medical condition or are on medications.)
3) Weight Lifting. Lifting heavy weights results in extremely transient blood pressures and one study found that long term weight lifters had higher arterial stiffness that normal.  I cover this and other related issues in more detail on my link on Weight Lifting and Your Arteries.
4) Salt. The more sodium that you consume, the higher the "plasma volume" in your arteries and vessels. This can lead to high blood pressure, especially in sensitive individuals. However, salt-sensitive or not, it appears that everyone is susceptible to accelerated arterial damage from high sodium consumption.  Researchers are not completely sure why at this point, but preliminary evidence points to an enzyme called MMP-9,  recently associated with arteriosclerosis. 
5) Diabetes. Of course, anything that raises blood sugar is going to lead to more of Advanced Glycation End Products, mentioned above, and accelerated hardening of the arteries. Diabetes is the ultimate blood sugar raising machine and so it is no wonder that diabetes is correlated to arterial stiffness. 
6) Smoking. Yes, sticking burning leaves into your mouth will damage your arterial walls. 
7) Moderate Alcohol Consumption. These improved arterial stiffness in one recent study.  Of course, moderation is the key here.
8) Blueberries. The equivalent of one cup of blueberries per day dramatically increased nitric oxide, lowered blood pressure and decreased arterial stiffness, at least according to one study on women. However, the results would very likely be replicated in men as well. See my page on Blueberries and Erectile Dysfunction for more information.
The bottom line is that if you want to be hard, you've got to keep your arteries soft.
1) J Am Coll Cardiol, 2010, 55:1318-1327, "Prediction of Cardiovascular Events and All-Cause Mortality With Arterial Stiffness"
3) Am J Hypertens, 2007 Mar, 20(3):242-7, "Advanced glycation end-products and arterial stiffness in hypertension"
4) Hypertension, 2005, Jul;46(1):232-7, Epub 2005 Apr 25, "Advanced glycation end products are associated with pulse pressure in type 1 diabetes: the EURODIAB Prospective Complications Study"
5) Hypertension, 2005, 45:349., "Arterial Stiffness and Hypertension: A Two-Way Street?"
7) Am J Hypertens, 2007 Mar, 20(3):319-25, "Sodium, arterial stiffness, and cardiovascular mortality in hypertensive rats"
8) J. Nutr, 2011, "Sodium Intake Is Associated with Carotid Artery Structure Alterations and Plasma Matrix Metalloproteinase-9 Upregulation in Hypertensive Adults"
9) Eur J Clin Invest, 2008 Jan, 38(1):24-33, "MMP-9 haplotypes and carotid artery atherosclerosis: an association study introducing a novel multicolour multiplex RealTime PCR protocol"
10) Diabetic Med, 1992, 9:114 119, "Arterial wall compliance in diabetes"
11) J Am Coll Cardiol, 1993, 22:1881 1886, "Short and long-term effects of smoking on arterial wall properties in habitual smokers"
12) Nutrition, Sep 2013, 29(9):1122-1126, "Acute effects of beer on endothelial function and hemodynamics: A single-blind, crossover study in healthy volunteers"