Why does the poet repeat the words “for ever” several times in lines 22-30 of "Ode on a Grecian Urn"?

First, there is no way that we can know precisely why Keats wrote the words he did. Given that time travel is not possible and he did not write an essay explaining his intentions, all we can do is discuss how the choices he made (whatever his reasons for making them) may contribute to the effect the poem has on its readers. 

The central contrast in the poem is between the eternal present of the scene captured on the urn (probably in reality the Parthenon frieze, not an urn) and the fleeting nature of real life. In actual life, as opposed to art, a musician performs for a set time and them stops, leaving only with memories. Young people age and die (Keats was dying of tuberculosis). Trees shed their leaves every season and then eventually die. In contrast, these images on the urn remain intact and unchanging "fore ever". It is this durability that the repetition of the word emphasizes.