How does the speaker try to reassure his beloved in the first stanza of John Donne's poem 'Song'?

In "Song", the speak reassure his beloved by saying that he is leaving neither because he is tired of her nor in the hope that he may find someone who is better fitted for him (by "nor in hope the world can show a fitter love for me", the speaker actually implys that as far as he is concerned, his beloved is the best for him). He is leaving because someday they have to be parted by death. His leaving now provides a chance for his beloved to feel what it is like to be parted from him someday in the future by death.