Do all cells in the human body undergo mitosis? why?

Nope; there are different types of cells in the body; for example somatic and germ cells

Our body cells are somatic cells. These cells must go through to maintain daily functions of the cells like growth and repairing damaged tissue.

The other major type of cells are germ cells. Germ cells complete in the reproductive organs to create reproductive cells (ie. sperm and egg cells)

These cells must go through meiosis because the final result of meiosis will lead to a cell with half, or the haploid, number of chromosomes at the end of the process. Haploid cells will fuse together during fertilization to create diploid cells.

Our body cells are diploid cells. This means that they have two copies of each chromosome set, one from mom and one from dad.