I am very sorry to hear about your father. It sounded like you had a very special relationship with him. I pray that you find comfort in the memories that you hold dear. Don't feel like you're stepping out of line to question WHY this happened. Many faithful servants of God in the past wondered why He allows suffering. King David, Job, Asaph and Habakkuk struggled with this question, and God not only recorded their stories, but also answered their questions. God's purpose in recording these events was so that we can have comfort and hope. Romans 15:4 says, "For all the things that were written aforetime were written for our instruction, that through our endurance and through the comfort from the Scriptures we might have hope."
I don't intend to sound preachy, but having lost someone close to me in death as well, I have found real answers and comfort in God's Word. I just pray that I can share some of it with you.