You eat too much. ``How could I have finished that entire pizza?'' ``Those doughnuts cried out, `Eat me!'''. When what goes in exceeds what you burn, your body has left-over nutrients floating around in the bloodstream.
We evolved in a world where the normal conditions of life were hunger and cold. On those rare occasions the body enjoyed a feast it, like the prudent squirrel, made provisions for the hard times that would surely follow.
Fat cells are the body's equivalent of a piggy bank. Fat cells sit on the banks of the bloodstream and, whenever they see excess food, snatch it out and build molecules of fat to stuff in their little cellular storehouse. Each fat cell is, in essence, a little rubber bag: when it sees too much food it snarfs it up and expands.
When this goes on, the larger rubber bag expands: you gain weight.