as a cell grows, its volume increases faster than its surface area, so it becomes harder for the cell to maintain homeostasis (harder to expel wastes and harder to obtain nutrients). because of this the cell must either stop growing or divide.
cells also divide to replace old or damaged cells.
The stages in a cell's life are called the cell cycle. This works a bit like a washing machine cycle - each stage must be finished completely before the next one is allowed to start. Instead of wash, rinse, spin and drain, the cell cycle is made up of stages in which the cell grows and rests, copies its DNA, and divides into two new cells.
There are many different sorts of cells but all have roughly the same cell cycle. However, the time taken to complete it can vary enormously. The cell cycle of a fly embryo cell takes just eight minutes while a human liver cell cycle can last longer than a year. Some cells can withdraw from the cell cycle and rest before re-entering it. Many cells in growing embryos often skip the resting stages altogether. Cells that have stopped dividing, like brain cells, never re-enter the cell cycle.