I made an account specifically so I can weigh in on this topic, as I have just written a highly in-depth analysis on this poem by Dylan Thomas. While Andrew Martin Is absolutely correct, it goes much deeper than that.
Through much research on Thomas’ youth and the relationship he had with his father, I was surprised to find out that he actually had a horrible relationship with him and wanted to get far away. Yet he still wrote a beautiful poem for him on his death bed, one that is often quoted at funerals to this day.
When analyzing poetry, one must consider who or what is the speaker, and what is it’s intention. For this particular poem, the speaker is the author himself speaking to his dying father, begging him to stay alive and keep fighting. But who does Dylan Thomas think he is? He resents his father and leaves home at an early age and doesn’t see him for YEARS...then thinks he can just walk in on his father’s last peaceful moments?! The words have conviction, but the speaker of the poem and his intentions are ones coming from selfishness.
Yes, the poem is still about not going down without a fight, but consider the person saying it: for their own gain.
Now if you rewatch the movie, you will see only two characters speak the poem, Dr. Brand (Michael Cain) and Dr. Mann (Matt Damon). Oh, what’s that? The two most selfish characters in the entire movie... and so the plot thickens!