"I don't feel sorry for those who lack the discipline to eat more." - J.M. Blakley
One of the many problems with our society is that people are too spoiled. They want things immediately and with as little work as possible. This applies to people who whine about how they cannot gain weight, no matter how much they eat, but usually they eat like a mouse, nibbling at their food. Imagine if these same people were forced to live in a third-world country or in a combat zone.
The fact is that if the guy trying to bulk up lifts like a madman in the gym, yet refuses to eat with the same zeal and effort, he's going to fail. If he refuses to ingest a surplus amount of calories from food after burning what the body needs for normal, daily functions of the body, along with exercise, sports, etc., his bodyweight will not increase. He needs to eat more than his body burns. It is a very simple concept.
I was always a very skinny kid, growing up. In high school, I weighed 100-110lbs. When I was in the U.S. Army, I weighed ~125lbs after returning from Iraq, in September 2003. I put on some pounds after I started training consistently again in 2004, then gained approximately 30lbs (145-175lbs) from 2004 to 2007. I really struggled to pack on the pounds, especially around 2004 and 2005, but after much frustration, realized that the "trick" was to eat massive amounts of the three macronutrients (protein, lipid, and carbohydrate) and calories in general, sometimes until I was physically sick, spending many nights sitting on the toilet. At that point, I began seeing much greater gains in bodyweight. It was very difficult, but I was successful because I stopped making excuses such as, "My metabolism is too fast!" or "I don't have enough time to eat!", etc.
I also came to the realization that to add any significant weight to my frame, I needed to eat things which most people consider unhealthy and what bodybuilders call a "dirty bulk", such as pizza, twinkies, lasagna, chocolate, burgers, fries, etc. I only purchased and ate food which contained the most calories, never wasting my time with anything that had the words "low carb", "diet", or "light" on the package or wrapper. Plus, I ate more often, whenever I had free time, if possible, and there were plenty of times that I tried to exceed my threshold of feeling full, pushing past that feeling. Also, when I was younger, I did not like butter, but now I put butter on my bread on a regular basis. Little things like that can make a difference, combined with other changes.
During my quest for more mass, I read a few things by J.M. Blakley, and one tip that really helped was to always drink liquids that have calories (i.e. milk, beer, regular soda, and tea) instead of flavored water, zero calorie soda, or seltzer. This tip was not some complicated formula or anything like that, but I just never thought that the calories from drinks would make much of a difference.
If you want, you can make eating a personal challenge, like lifting in the gym. Tell yourself, this time, you're going to eat one more slice of pizza, or a few more ounces of steak, or at least slightly more of any particular food, compared to last time. Break personal records at the dinner table or restaurant like you would at the gym.
At 5'5.75" and 175lbs, I'm still not done gaining, but I'm slowly achieving my size goals. I have a small bone structure with a 6.5 inch wrist. If I can do it, YOU can too. Now grab a fork, knife, spoon, chopsticks, or just your bare hands, and EAT IT!