The new Das Boot series triggered this question with me. If you were assigned to a u-boat, did you know your life was probably over? Or was there simply no way of knowing, as a subject of Nazi Germany?
The thing about that statistic is, it doesn't apply anywhere close to evenly over time. Here's a list of U-boat losses by year.
| 1939 | 9 |
|---|---|
| 1940 | 24 |
| 1941 | 35 |
| 1942 | 86 |
| 1943 | 241 |
| 1944 | 234 |
| 1945 | 136 |
As you can see, the German submarine service suffered dramatically heavier losses late in the war than it did in its first few years. This is reflective of the realities of the Battle of the Atlantic. During the First and Second Happy Times (roughly the second half of 1940 and the first two-thirds of 1942, respectively), the U-boats inflicted very heavy damage to Allied shipping, but at a growing cost to themselves. But as the Allied learned to combat the underwater threat, the paradigm gradually reversed. Throughout the last two years of the war, U-boat casualties went up as Allied shipping losses went down.
It is also worth noting that the number of boats at sea at any given time also fluctuated. Only about forty were available for service at the beginning of the war. This ultimately rose to a war-time high of nearly 160 boats at sea in early 1943. While the large numbers of boats that entered service in 1941-1943 somewhat made up for the heightened loss figures at this stage, by 1943-44 the U-Boats were taking dreadful casualties that accounted for the horrendous percentage you quoted.
All this is to say that the experiences of two U-boat sailors could be wildly different. A sailor in 1941 would not be faced with anything like a 75% loss rate, and so would have much less reason to be troubled than a sailor in 1944.
Follow up question: Was there any sort of limit on the number of tours a u-boat crewman would be required to go on? Similar to how US bomber crews were sent home after X combat missions. Or would a crewman keep going out to sea until he died or the war ended?