Most young people who join the military do so for only one enlistment (four years.) If they qualify (and most do) for reenlistment, the military makes an effort to get them to "re-up" (reenlist for another four years, or more.) The reasons for this effort are pragmatic. It takes $10-15,000 to recruit a new soldier, and four years of experience is a valuable thing. Moreover, the NCOs who supervise day-to-day operations come from people with at least 5-10 years of military experience. So the military seeks to 40-50 percent of first enlistment troops to reenlist. But sometimes, as with the navy right now, the reenlistment rate goes way up. Currently it's at 57 percent. The brass have let it be known that the re-enlistment rate should not go any higher. The solution is simple, the navy just raises the standards for reenlistment. That way, the better qualified people get to stay in, and ten years down the line, the navy will find itself with an above average generation of CPOs (Chief Petty Officers, the people who REALLY run the ships.) This also has a dark side, for such a class of excellent CPOs is inevitable followed by an average one. At which point, the old timers (both officers and NCOs) will wistfully pine for the good old days.
Patriotism, and two years of recession at home, have made it easier for the U.S. Navy to get sailors to re-enlist. The navy has become too successful at this, and that's a problem. If too many troops stay in, the promotion prospects for all of them become worse. As a result, the valuable veterans of 10-12 years service tend to get fed up with the poor promotion prospects and leave the military. This is very bad.