People suffering from suicidal thoughts and deep depression are basically mentally ill - and their brains and thought processes are unbalanced due to chemical imbalances in their brains.
As a result, logical trains of thought, and the consequences of our actions - that those of us who are not suicidal, carry out - are not able to be carried out by the suicidal person, because thoughts of suicide and the constant outlook, that there's "no other way out", dominate their thoughts, to the exclusion of all else - including the consideration of what may happen when they bale out of a flying aircraft and leave it to crash.
Unfortunately, suicidal tendencies tend to be hereditary and run in families - and people who have suicidal tendencies regularly fall into deep depression and try to take their lives - as their disordered thoughts see no way to move forward out of their depression and their "need to end it all".
I have personally known a couple of families who had several family members over several generations, develop suicidal tendencies - and I have personally known several people who suffered from suicidal tendencies, who regularly attempted to take their lives - and who eventually succeeded in doing so.
It's important to develop the skill to be able to recognise when someone is suffering from suicidal tendencies and the attendant deep depression, and to get help for them.
I'm not talking about the occasional bouts of depression that a large percentage of people get, when they feel overwhelmed by a constant run of adverse life events - and who always manage to climb out of that depression.
I'm talking about the people who regularly speak of suicide as the only course of action, and who fall into deep depression, that they cannot shake off, or be cheered up, and talked out of it.