This poem is about a man's telling his beloved not to mourn for him, or have any outward display of emotion about this leaving because their love is greater than that kind of display. He is very reassuring to her about their love and how it is greater than "ordinary lover's love." A great thesis statement would express that kind of statement and then state that John Donne uses many metaphors drawn from lots of different areas of knowledge to prove how their love is better. His metaphors are quite unique! In this poem he talks about the planets, about the chemical properties of gold, about how he and his wife are like a compass (used to draw perfect circles), about how ordinary lovers can't be apart because they rely on physical presense while he and wife do not. You could definetly talk about how he reassures his wife through the use of all these metaphors. One common mistake with this poem is that the speaker is talking about his death -- it is more likely that Donne wrote the poem because he was leaving on a business trip and had to leave his wife at home. The last three stanzas specifically talk about his returning to her. The first two stanzas do reference death, but he is talking about the calm death of man and he is telling his wife to act that same way when he leaves -- act calm, not full of outward displays of emotion.