Here's why:

* Gametes are haploid cells, meaning they contain only one set of chromosomes (n).

* Fertilization is the fusion of two gametes (usually a sperm and an egg).

* When these two haploid gametes fuse, their chromosomes combine, resulting in a cell with two sets of chromosomes (2n). This cell is now diploid.

This diploid cell is called a zygote, and it marks the beginning of a new individual's development.