Here's why:

* Diploid-dominant: The dominant stage in this life cycle is the diploid form, meaning the cells have two sets of chromosomes. This is the case for humans, where the vast majority of our life cycle is spent as diploid organisms.

* Haploid form is unicellular: The haploid form (gametes, sperm and egg) is only unicellular. It exists briefly during the reproductive process.

Other life cycles:

While humans have a diploid-dominant life cycle, there are other types:

* Haploid-dominant life cycle: The dominant stage is the haploid form, which is multicellular. Fungi and some algae exhibit this type of life cycle.

* Alternation of generations: Both haploid and diploid stages are multicellular and alternate during the life cycle. This is seen in plants and some algae.

In summary: Humans, like most animals, have a diploid-dominant life cycle where the haploid form is reduced to a single-celled stage, the gametes.