It seems the issue is accelerated when consuming alcohol because the liver tends to deal with the alcohol entering the system first bypassing the fat that there is to process. On top of this people tend to eat more calorie rich foods when out drinking - think of the burgers, chips and other fatty, salty snacks that always taste oh-so-good on a night out. As the years tick on we also lose our teenage metabolism, but not so much the appetite so this only helps to complicate matters.
Want my advice: swap out those butties at lunch for a chicken salad if you know you are going to have a beer later. And (more importantly) dust off that gym card once or twice a week or go for a brisk walk, you can always reward yourself with a cold one!