I met the wife of a biker friend of mine on Saturday and simply couldn't believe how she looked. I last saw her about four years ago when she must have weighed about 80kg, but now is about 45kg.
I was astounded and she looked fabulous, actually. However, I didn't feel it was right to ask her about her weight loss, apart from saying to her that she looked great.
I had lost some weight 10 years ago and everybody was telling me how fabulous I looked. A close female friend also added "it was about time!". Imagine what I think about her and her comment now that I have gained it all back 
I once tried to return the compliment to a lady in the town where I grew up, because I remembered how it make me proud that people noticed the difference when I followed a strict diet. I was already living in Argentina and was back in Italy for a visit. She was a round red-headed lady all her life, with a big round face. I found her slim, her face now not round at all, with her long red hair... she looked stunning, and I complimented her on her weight. She told me she was sick. She died a year or two later. Since then, I shut up about people's weight, unless they bring up the subject. It was quite a lesson. 
I am the same height as GlasgowJohn, and once I went up to 89 kilos. I was a teenager, and a fairly depressed one, and I ate the world.
I was 83 during university, and 77 when I started working. I am now close to 80 (I no longer dare to weigh myself) and I have joint aching, back aching, I don't sleep well and I feel sluggish etc. The lowest I ever got was 74. I would be happy with a steady 70, but I really enjoy food, and I loved eating incredible amounts of food since I was a newborn, as my mom has always confirmed. She said that other moms complained about their children not eating enough, and she couldn't understand why. Her issue was finding ways make me eat less.
I also get pretty grumpy when I am hungry, as my husband knows.
For me, the hardest part of a diet is the feel of hunger. It drives me mad. When I followed a diet, I had to make a mental shift and consider as "poisonous" all the food I couldn't eat. Psychologically speaking, it didn't feel a healthy approach, but it worked.
However, I usually get to a point where I think I can have a slip or two, that I am never to return to XX weight now... except that the weekly exception becomes a daily slip and a few months later I have 5-8 kilos more. 
