As we all know, the reason is so simple - too much of the wrong types of food and not enough exercise to consume the intake! However the rectification of this can be a real challenge.
Got serious three years ago. Gym at my work, three mornings a week, starting at 6:30 - alternate cardio and weights. I was 93kg, I wanted to be 83kg. Now I'm 100kg! Trust me - it's all muscle!

Well, it truth it is due to that, but I still carry 5-10kg around the stomach that is a real challenge to lose now I'm in my 50s (early 50s! - I'm not old!!). I weighed myself after every gym session. My weight dropped into the high 80 kgs, then turned around and went up again to the high 90s and stayed there for 2 years. I've just increased the weights and after a couple of months I've cracked the "ton". But my clothes (waist in particular) fit well for trousers that were too tight some years ago. I am struggling a bit with shirt collars.
But, oh the challenge of keeping it up. I regularly have to swap mornings due to work commitments in different locations. A 6:30 start means leaving home at 5:15 - a real struggle after a late night to midnight or beyond!
Food wise, I eat well, but not enough fruit. One small chocolate bar as a treat every few weeks, no fast food (McDonalds' salads only), no softdrinks, no icecream, few sweet desserts. My weakness for cakes and biscuits means I have to have none in the house! By the way, fortunate enough to not drink nor smoke - that helps with the health and weight. I do drink far, far too much coffee (and strong coffee - come round and I'll make some Arabic coffee for you!)
So, I'm overweight by any measure - but doing fairly well (so says doctors, cardiologist and chiropractor) and look like keeping it that way IF I can keep up the willpower - that's the challenge.