just wondering how come you should change the oil every time you get back from a ride or a couple of rides
Because oil breaks down with use and loses its ability to lubricate the engine properly. So if you do not change the oil regularly with new oil the engine will wear faster.
Dirt bikes are high performace machines and are hard on oil. They dont hold a great deal of oil volume so they will reduce the oil's lubricating ability faster than your average car or whatever.