Why does the poet want to go away from his beloved in "Sweetest Love, I Do Not Goe"?

The speaker wants to go away from his beloved as a "test run" for the separation they will experience at death. Since it is very likely they will not die at the same moment, one of them will be left alive and alone, and they both might as well get used to the idea. Travel, which will part them, is similar to a "feigned" (fake) death. The speaker says:

Thus to use myself in jest / By feigned deaths to die.

He goes on to state, however, that just as the sun sets only to rise again, he will return. In fact, it is more likely that he will return than that the sun will rise because he has motivation to do so, while the sun simply rises without "desire nor sense."
He also notes, however, that humans have little control over what might happen to them. Nevertheless, the speaker ends on an upbeat note, for whatever happens to the couple, the fact that they keep each other alive means that can't ever really be parted.