The poem is the speaker's ode to his beloved. In the first stanza, the tone is celebratory. The speaker compares his love to a rose at the height of its vibrancy and color; this is why "red" is repeated. In the second stanza, the speaker praises his beautiful "lass" saying that he will love her until the seas go dry. The tone is still celebratory. But there is just a subtle hint that his love is not absolutely eternal. Again in the third stanza, the speaker says that he will love her "While the sands o' life shall run." This could mean for the rest of their lives or until the end of time. Critics suggest that this means he will love her for a long time but not truly forever. This seems like a unfair criticism until we consider the final stanza where the speaker is leaving her. In this case, he is expressing his love prior to leaving. Therefore, there are many different interpretations of just how long his love will last.
And fare thee weel, my only Luve!
And fare thee weel, a while!
And I will come again, my Luve,
Tho' it were ten thousand mile!
The poem is a celebration/declaration of love, so the tone is positive. But as the poem concludes, a tone of melancholy sets in because the speaker is leaving his beloved. He is celebrating and/or justifying his love before he leaves because he wants to communicate that is love for her can not be diminished by a separation of space or a duration of time.