The transition lets you say goodbye to your fertile phase.
Unlike men you stay as a woman is not fertile into old age. After the fertile phase, a woman's life has even a stage where you do not have to pass. Longer life
To make those adjustments have your body and mind to resist. Lot of work
In short, the fixed (usually) monthly pattern you knew hormones during the fertile phase begins to change. Years before the last menstrual period
The production of estrogen and progesterone by the ovaries lose the old pattern and will eventually decrease so far that your periods stop permanently.
For few women, this process of transition a peaceful and gradual process. Much more often it is accompanied by peaks and valleys in your hormone balance and gives the different physical and mental symptoms.
In the early stages of the transition, you can even increase estrogen. Estrogen is often only falls short of your actual menopause (= the day of your last menstrual period)