Why do people have nerves break downs and go crazy?

Some people have less of an ability to cope with stress than others. And it is not their fault.  What stresses out one person might not stress out another, and what doesn't stress out another might stress out the other. Sometimes people are put into highly stressful situations. For example The supervisor in charge of the department that made hydraulic pumps for nuclear missles and aircraft. Was running behind on orders and his workers were not getting it done. He was under major pressure because this stuff is so critical. One day it was too much and he crawled under his desk and would not come out. He stayed under his desk for several hours while people tried to talk him into coming out. Finally they got his wife and she was able to get him to come out. He had been doing this work 20 years and he never worked again. Very sad story.....

You are on the right track when suggesting stress or hormones. What happens is that the stress alters the amount of certain chemicals in the brain, such as serotonin, cortisol, norepinephrine,  etc.  The altering of these particular brain chemicals causes depression and anxiety. If the chemicals become too unbalanced, that is when the "breakdown" occurs. The person cannot help it themselves, they need medical attention. There are medications that can restore the proper balance of the chemicals.
There are actually several different things that may cause the initial stress...death of someone, past abuse, conflicts with friends or family members, genetics, major events such as divorce or losing your job or even getting married or moving, illnesses, drug abuse....but whatever the initial stress is, the resulting chemical imbalance in the brain is the problem. It has nothing to do with the strength of a person. We will all face stress and challenges in our lives, and any of us can become chemically imbalanced. Hopefully if it happens to you, that you will see the warning signs and symptoms as they are starting out, and get proper medications for yourself before a breakdown occurs. Never be afraid to discuss feeling depressed with your doctor. The medications really do work. I have a genetic medical condition, and I take 2 medications... Effexor and Wellbutrin. And I feel great and am happy all the time now.

IM not sure but when my mom had one, she cut off all of her hair

It happens because some have really a problem in the nerves. Some can't withstand pressure.

Some people have hard time to cope with their stress and problems thats why they will go nervous break down and directly go crazy.

Many people have nervous breakdowns and this is natural especially with women who have had hysterectomies...because CERTAIN amounts of hormones are missing and there is a hormone in balance. Also some people put MORE pressure on themselves than the average Jane or Joe Doe. Those people have higher aspirations and immediate satisfactions and when things happen out of the ordinary or NOT as scheduled, the person breaks down.
Empty nest syndrome also causes nervous breakdowns.
Weather can cause nervous breakdowns...look at Hurricane Katrina victims.
I don't believe that weakness has anything to do with it. The stronger person may have the nervous breakdown cause whereby, the weaker guy goes to work, buys beer and returns home to drink the whole case alone, he settles his issues in the beer can. The stronger person COULD be one who is constantly task oriented and keeps goals where things are built and housing improvements are made and possibly OCD. This person may sleep 4 hours a night, have a very physical job and exert tons of pressure of her/himself and not drink or smoke.

I think nervous breakdowns are prone for anyone...and the more pressure a person can take and carry on with, then, eventually that person may crack.
On the other hand, it can be inherited. I have seen that several times.

A person is in control of their life. Every thing is managable and things are going the way they think it should go. Then something happens (an event) that makes things go into an unmanagable direction. The control in that person life starts to slip. At first the person thinks they can take this unmanagable situation and try to manage it, but over time (short or long) they get to a point that 'this is too much, that is too much' and they can not control the un-control any longer. This is when they give over to the un-control and feel lost inside and become clueless. Their mind breaks down of rational thought toward themselves or others. That un-control is their new world of control.