Both my paternal grandparents died of alcohol related illnesses.
My grandma had done so much damage to herself from smoking and drinking that she could barely speak. She was so dependent that she couldn't stop or it would kill her - she was too unwell to go through medical rehab so she just stayed home and drank. She didn't know who I was, didn't know what day it was or even where she was half the time.
My granddad, somehow, managed to remain functional into his nineties when he died of prostate cancer. He was (on the surface) intelligent, happily married and had a large family who loved him - but the reality was there was constantly whisky on his breath, he couldn't walk in a straight line and he regularly turned up to work drunk - though because he was so "tolerant" of it, it didn't show to anyone who didn't know him well.
I also had an acquaintance who drank herself into oblivion, who went through multiple detoxes and who regularly shit and pissed the bed because she couldn't get up in time. She would vomit down herself and keep drinking. She was constantly in and out of hospital and died of multiple organ failure in her fifties (while looking about 95). 10 years previously, she seemed perfectly normal - loved a night out, looked her age, was happy and social and held down a job. It happened scarily fast.
I think when you've seen your loved ones go through that, and have seen the impact it has on your other relatives, you can't bring yourself to have a casual attitude to alcohol.