As many of us have learned from sad experience, it takes very little excess food input on a regular basis ('excess' both in caloric terms, and in relation to energy output) to begin to store excess pounds. All you have to do is eat just a few hundred calories a day more than you need (like the equivalent of one candy bar, or a few slices of bread) and be inactive with your body, and you will eventually add weight. Once this habit is ingrained in you, it’s very hard to break it.
Specifically, it takes 3,500 additional calories to gain 1 pound of weight. That's about 3 extra plates of food, or 14 big sodas (20 oz. size) or about 5 double-size burgers.
The good news is, even small reductions in eating, coupled with moderate increases in activity, can begin to help you lose weight.
Try this: just cut out one thing from your food routine and see what happens. Stop drinking sodas. Substitute flavored seltzers, or iced tea, or best of all WATER instead. Or cut out desserts, except once a week on Saturday night if you must. I'll bet you lose some weight. No severe crash diets necessary. Just a slight change. A little at a time works best.