This sonnet by Shakespeare talks of one incident that occurred between the speaker and his mistress where she said to him two words that destroyed his life. The words "I hate" were so strong and so powerful that they immediately resulted in the speaker entering a "woeful state." That the speaker is completly fixated on his beloved is indicated by the way that in spite of the words that she is saying, she still has lips that "Love's own hand did make." However, in spite of this unpromising start to what she has to say, she sees how her words have wounded the speaker so badly that her tongue, so used to giving "gentle doom," changes to have a very different ending.
Just as beautiful morning follows the fearsome night, so the mistress completes her sentence by saying "not you," therefore saving the life of the speaker, as he puts it. The sonnet clearly describes a relationship where the speaker is hopelessly in love with his beloved and unable to think rationally. He is therefore in a very vulnerable position, open to the whims of his mistress and able to be wounded as she wills and desires.