Why did hashem make it so that people should live for a very long time back then? But, today our life-span is 120 years. Why is this?
Originally, humankind was destined to live forever. Due to sin, death was decreed upon mankind.
Yet, the initial generations still enjoyed long lifespans, averaging approx. 950 years. This is possibly because while they became susceptible to death (by disease, natural accident or intentional assault), men did not experience ageing. They continued to live healthy lives, and to have children, until they died. According to current research, if we take away the effects of ageing, people would live to an average 1,400 years old, which is not far from the biblical account.
However, at the time of the flood Hashem punished humanity again, and the years of man were reduced to 120. This did not happen instantly, but gradually, and only after some 16 generations (from Noah to Moshe Rabbeinu) did the effect fully take place. Yet, the process of ageing (which is today known, or at least thought to be genetic — tinkering with genes of small mammals has succeeded in doubling their life expectancy) set in immediately, and life spans began falling, together with people becoming weaker in old age (and unable to bear children).
The end of this process gave rise to the situation we know, whereby the process of ageing limits lifespans to no more than 100-120 years.