While suicides cause only a small fraction of the annual marine deaths (training and off-duty automobile accidents kill far more), they are indicative of stress producing problems. The marines are investigating past suicides in more detail, to see if theres anything that can be done to reduce the rate, or the stress that leads to the suicides.
The marines are by no means the military force most afflicted by suicide. The Japanese armed forces have a rate of 39 per 100,000 troops. In the Russian army, the rate is over 60. Suicide rates are heavily influenced by cultural factors, as well as local ones, like the atmosphere in an armed force.
The U.S. Marine Corps is a very effective and demanding organization. Lots of performance, lots of stress, lots of suicides. The marines average 14 suicides a year per 100,000 troops. The U.S. Army, in contrast, has a rate per 100,000 of about 11. This year, the marine rate is up to about 17 (not all suicide deaths are confirmed, and some turn out to be accidents or homicide.) The previous peak year for marine suicides was 2001, when the rate was 16 per 100,000.