Why do all normal cells in human body have same number of chromosomes?

Females have two X chromosomes in their cells, while males have one X and one Y chromosome. Mitosis; before it divides it replicates & meiosis; sex cells have half the amount of chromosomes that a normal cell has so when they join another, there are 46 chromosomes in the cell.

The reason as to why all human body cells have the same number of chromosomes is because of the process of mitosis and meiosis. 

Mitosis is the process of cell division in which body cells or somatic cells divide to yield two daughter cells with the same number of chromosomes as the parent cells. The process involves a series of steps, and results in the formation of two new nuclei each having the same number of chromosomes as the parent nucleus. The process ensures that have the same number of chromosome. In meiosis, sex cells have half the amount of chromosomes that a normal cells has and therefore when they join another, there are 46 chromosomes in the cell.