I never understood how someone could have a difficult time putting on weight until I had my daughter. Growing up it was super easy for me to put on weight. I played football in high school and as soon as I started lifting and taking protein supplements I added weight very quickly.
My daughter has always struggled to put on weight though. Partly because of how active she is, partly because of her metabolism, and partly because of the types of foods she enjoys eating. It's a true chore for her and a constant struggle we've had with her to help put on weight. I'm now more sensitive to these super tall athletes that have a hard time putting on weight, because for some people it truly is a struggle.