Although the speaker of “Do Not Go Gentle into That Good Night” is fully aware that we will all die eventually, he urges us nonetheless to defy death for as long as possible. Instead of calmly accepting our inevitable demise, we should “rage, rage against the dying of the light.”
That is the approach that good men adopt when they come to their last few moments on earth. They don't accept their deaths, because they know, like the speaker, that death is something negative, something that we should try to avoid as long as possible.
Wise men, wild men, and grave men also understand the importance of defying death to the bitter end. The dying of the light, a metaphor for impending death, is most assuredly not a good thing, and so it cannot be accepted. Yes, wise men at their end know dark is right, but that doesn't mean that death is somehow less negative as a consequence. And so they “rage, rage against the dying of the light.”
The speaker hopes that his father will follow the example of the men he's already cited. He urges him to curse or bless him with his fierce tears. The word “fierce” is important here because it illustrates the attitude that the speaker so much wants his father to have towards his impending death.