Why is the standard deviation of a portfolio usually smaller than the standard deviations of the assets

Question:

Why is the standard deviation of a usually smaller than the standard deviations of the assets that comprise the
Portfolio
A portfolio is a grouping of financial assets such as stocks, bonds, commodities, currencies and cash equivalents, as well as their fund counterparts, including mutual, exchange-traded and closed funds. A portfolio can also consist of non-publicly...