Why does the poet want to go away from his beloved

I inferred you're referring to the story "Sweetest Love, I Do Not Goe" by John Donne

The speaker wants to go away from his beloved as a "test run" for the separation they will experience at death.

Explanation:

In this story piece, it is strongly believed that the speaker goes away in other to experience what was to come– their death.

Remember also, that they will not die at the same time, so test running their separation now would prepare them for a time when one of them will be left alive and alone.

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explanation:

i have a basic answer but if you want a more in depth