Just like ブリトン said, some situations can damage a person to a degree that is beyond repair.
In a way, one can compare the mind to a muscle. When we go to the gym for the very first time we stimulate the muscles in a way they were not used to. Then we feel sore the next day, but with some consistency, proper rest and nutrition the muscles become stronger and stronger each day. Now let's imagine another person goes to the gym for the very first time, spends 6 hours a day working out, (because "no pain, no gain", right?) without proper supervision, nutrition or rest. This last person will certainly damage the muscles instead of getting stronger. Not to mention that even if the second person would be following the first person's exact same workout program they would be likely to get completely different results, because each person has his individual traits.
Of course it will vary from person to person, but I strongly believe most situations can make you emotionally stronger once you train your mind to deal with it with the right mindset. Just like physical training, some people can easily convert external stress into internal stimulus whereas some need the help of a professional in order be emotionally prepared for the difficulties in life.
While I agree that we can learn from the mistakes of others, I think learning from our own mistakes can also have a strong impact on how we change the course of our lives after a bad decision.
By the way, are you ok with receiving grammar corrections from others? (You and anyone else here can feel free to correct mine...)