It's complicated and many factors come into play. I think the most important reason is most pre meds do not know what they are getting into and/or do not know how the journey would affect them. They think they want to help people, maybe have seen some sick people suffer or family member suffer and therefore wanted to be a healer, or maybe they were interested in biology/science and that's why they decided to do medicine. (Big mistake for many people). After that, they have to work really hard since college (some even HS to get into as good a college as possible). In college they have to work hard to maintain a decent GPA, while doing extracurriculars/research to pad your resume. Senior year is spent filling out applications, writing essays, and going away for interviews. This takes away a lot of time from personal use starting from college. Many students spend their summers doing research, or studying for MCAT. Some even take gap year/years to boost their resume after college. I went to a ivy college, the recommended gpa is at least a 3.7 to have a good shot at getting into med school. (i believe it was 80% chance or so). A lot of people end up not making it (due to weed out classes) and change career paths in college.
Once the clueless pre med gets into med school, a whole new beast starts. You now officially a grown man/woman, but has zero power and is essentially everyones B****. Your existence is pretty meaningless to the hospital aside from the 60k tuition you pay to them per year. You get drowned in debt with interest that starts to grow from the day you take out the loan. You are spending all day/night memorizing useless information while your non med friends are making money and enjoying their best years. Especially in the beginning , it's not uncommon to get completely overwhelmed by the amount of information you have to know. A 6 month class in college is taught in 1-2 weeks in med school. (You also dont get the vacations that you get in college, like winter/summer/spring break) . You spend about 80 hrs a week studying. Then year 2 comes and you still studying for your classes, and on top of that, there's the step 1 exam that will determine your life so you are studying for that too. If you haven't stopped going to the gym or eating healthy or cutting into your sleep, nows the ideal time.
Then year 3-4 comes and you are 6 figures in debt. You've probably either gained weight from stress eating, or lost weight from not eating. You also look like ****, like you've gained 20 years, from not going to the gym and not sleeping. You've lost all contact with non medicine friends and the only thing you talk about with your 'new' friends is medicine cause that's all you do during your waking hours. Year 3-4 can be better or worse depending on the student. You are now working 60-80 hours a week in the hospital + you are expected to study after work so you dont look stupid at work, and for the exams at the end of each rotation. It's usually now that med students realize how useless they are. You only get in the way of the residents/attendings. The nurses, and techs treat you like garbage and you may be asked to do scut work. On surgery for example, i got to the hospital at 330AM, and left at 7PM on average. Somehow after that i have to eat, read, and sleep. All i did during work was present patients before OR, then retract in the OR. The environment was pretty crappy cause no one was happy from being overworked. A good day is when you learn more than a couple of things. You start thinking if this is really a good use of 60k tuition a year. Then 4th year comes and you realize you have to make the life altering decision of which residency to apply to. If it's competitive, hopefully you were doing research on top of everything mentioned above. Either way now it's time to study for Step 2 CK and CS which costs 2k total and is a complete waste of time, but still very stressful.
Residency comes and you realize you made your decision of which residency to go into solely based on 1-2 months of rotation. You realize being a resident is nothing like being a med student. Hopefully you at least dont end up hating your choice. But you are still working 80 hours a week, now with more production pressure. Your decisions actually matter! You start making a salary and realize if you live frugal, you can cover the interest on your 200k+ loans. You are still low on the totem pole and the nurses are still rude to you. You are now 26-29 years old and the only thing you have to show for your lifelong work is your mountain of debt. You are still eating crap, barely sleeping, and barely working out. Your non med friends are already buying houses and getting married and having kids. You ask yourself again why you decided to go into your field and torture yourself. 3-8 years later you are jaded, you dont find things exciting anymore cause you are tired and you've seen them many times already. You aged 50 years and your joints hurt. You are still drowning in loans, and then you realize the jobs out there suck. You probably have to move across country again to find a decent job. Then you ask yourself if you should do a fellowship, and if you do hopefully you did some research in residency. Then finally when you are done at age 35, you realize you finally need to get your life on track. You may want kids, a house, start paying off those loans and maybe save for retirement.
TLDR
1) I think mainly because of the imbalance between reality vs expectations. People go into medicine expecting one thing then finding out its completely different. You work 80 hrs a week for 10 years after college in a high stake environment, it slowly eats at you and drains you down. You are often working weekends, and get maybe half the federal holidays off. (since hospitals are open 24/7). You are so tired you stop working out, and dont have time to sleep and eat well. When you applied to med school, you were young and energetic. You probably told yourself you can handle 80 hrs a week no problem. Then reality hits and you realize it sucks and you can't do this for 10 years straight. The loans are also way higher than expected cause pre meds probably didn't do the math or didn't know how it worked. Most patients also dont appreciate you. Even after the 10 year of post college training, the salary is lower than expected, people also get bothered by litigations, increased regulations and bureaucracies, lack of freedom to practice , etc. Basically a lot of things you didn't know when you were a pre med.
And it definitely depends on a persons personality. But thats not something that can be easily changed. I know people who LOVES to work and dont mind working 100 hours a week til retirement. Some people may also be bad with deaths, but other people may be able to bounce back in 2 minutes and move on. Everything matters. Other people may have outside things going on that only adds to the stress. It may drive them to depression even suicide.