They don't "choose" to do it - they are assigned to do it,
though it is considered an honor to be assigned to the job. The
protocol for it (how the guards are changed, how they walk,
everything) was worked out many years ago.
The Tomb of the Unknowns, also called the Tomb of the Unknown
Soldier, is located in Arlington National Cemetery in Arlington,
Virginia. The memorial is dedicated to American service members who
have died without their remains being identified.