Unfortunately, I don't gain weight easily.
While there are many other factors that lead to weight gain, I think excessively consuming foods high in starch and sugar in combination with a sedentary lifestyle may contribute to weight gain.
Agree
I know how it is to not gain weight.
Ironically much of the same can be done to gain weight as to loose weight.
For instance: Plain sugar makes you feel "full" faster (though it lasts only a short time),
so protein is better than carbs to build a reservoir. That being said, carbs are an essential part of the diet.
Regular meals will both give a more stable blood sugar levels and make it easier to eat right.
(We all know we want unhealthy stuff when we are super hungry, eh?)
One should eat (ranked by amount)
Vegetables, grains (not white flour, but "whole grain"), fish, white meat, red meat and fruits + berries if available.