Do men need estrogen? The truth is that you need estrogen almost as much as the woman in your life needs testosterone." And she needs testosterone - believe me. It contributes greatly to her mood, libido, weight maintenance and insulin control just for starters. Of course, if she gets too much, it can cause her issues - just read about what happens to most women with PCOS.
Estrogen is analagous in many ways for males. We all know that overly high estrogen levels will likely lead to prostate cancer, andropause and loss of erectile strength and libido. You can read my links on The Dangers of Estrogen for more details. At the same time we very much need some estrogen for certain critical health functions.
So just how does estrogen fall too low in males? The primary way is in senior males with long term andropause. The typical pattern for an aging male is that his testosterone slowly declines by a percent or two per year after about his early to mid 20's. This decline in testosterone leads to slow weight gain, which in turn leads to increased estrogen levels due to the additional aromatization of testosterone from extra body fat.
However, as the years go by, it is quite common for testosterone levels to fall so low that even with the increased aromatization rates, a man can find that his estrogen falls to dangerously low levels. Thus, one could generalize and say that elevated estrogen is a middle-aged man's nightmare and decreased estrogen a senior man's.
NOTE: Estrogen actually has several different subforms. The most "dangerous" estrogen is E2, or estradiol. Typically, when one is talking about estrogen, this is the form being referred to. Sister molecules are estrone (E1) and estrial (E3).
Here are three key ways showing how badly men need adequate estrogen levels and how low estrogen can endanger a man's career, relationships and health:
1. Bone Maintenance. Estrogen is critically important for brain function, even in the men. Researchers have found that it is important for verbal memory in particular.  Estrogen is also essential for the growth of new neuronal connections and neuron maintenance.  Yes, testosterone has a profound effect on the male brain, but adequate estrogen is no less important.
2. Erectile Dysfunction. Results are preliminary, but it is clear that estrogen in males actually plays a significant role in nitric oxide production.  Of course, it is no secret that testosterone stimulates nitric oxide, but researchers were suprised to find that in men there were critical estrogen receptors that stimulate eNOS as well. Therefore, it may be almost as important to check estrogen levels as testosterone in aging males.
3. Verbal Memory and Brain Function. Estrogen is critically important for brain function, even in the men. Researchers have found that it is important for verbal memory in particular.  Estrogen is also essential for the growth of new neuronal connections and neuron maintenance.  Yes, testosterone has a profound effect on the male brain, but adequate estrogen is no less important.
4. HDL. Estradiol increases HDL and so it stands to reason that low estradiol levels could lead to low HDL levels as well. See my page on Estradiol and HDL for additional information.
Many experts emphasize that the criticality of the ratio of testeosterone to estrogen. For more information, read my link on The Testosterone-to-Estrogen Ratio, where I discuss how it changes over the years and even how Cialis can affect it in a positive way.
CAUTION: I had one extremely fit and athletic young man write in who was taking a variety of supplements and foods that decreased testosterone's aromatization to estrogen. This included zinc supplementation and green tea extracts (EGCG). His claim was that, because he was so thin and had very low body fat, that his estrogen was likely too low and leading to a variety of health issues. It may be something to consider for the young and fit.
1) Arch Intern Med, 2008, 168(1):47-54, "Endogenous Sex Hormones and Incident Fracture Risk in Older Me: The Dubbo Osteoporosis Epidemiology Study"
4) Med Sci Monit. 2003 Mar;9(3):RA63-8, "The nongenomic protective effects of estrogen on the male cardiovascular system: clinical and therapeutic implications in aging men"