"Pain is weakness leaving the body"
The above is a piece of propaganda used by the US Marine Recruiting office to get more people to join the marines. Its also figuratively true, and very effective in their commercials.
Pain is weakness leaving the body
If you punch a tree over and over again every day for a few years your hand wont be broken (unless you punched TOO hard). Instead, it will be toughed, and calloused, and you will be able to take far more pain.
When a problem in life is emotionally painful you are emotionally scarred. But if you learn from it, the emotional scars will scab over and you will be a strong, more experienced and mature person because of it. When this occurs, as long as your emotional pain does not destroy you, it will eventually make you strong if you allow it too.
If you run 15 miles daily for a year, your body will be in a lot of pain, especially the first month or two. By the end of the year, your muscles would be so broken down and rebuilt you would be very strong, provided you had enough nutrients, water, and rest during the year. If you tried to do the same with 50 miles a day, you would end up dead.
Pain truly is weakness leaving the body, provided that the pain inflicted is small enough that you can handle it and grow from it, emotionally or physically.