How do I get fat?

No, seriously, how does one get fat? I’ve been seriously thin since young and I have no idea what kind of condition I have. No, I’m not malnourished or anorexic. I eat a wide variety of food (a fairly large portion too), all types of meat, vegetables and fruits. I simply cannot plumpen up and it’s really getting to my nerves.

I’ve tried everything, eating more junk food or sweet stuff. I regularly snack on sweet stuff and it doesn’t do anything to me. I don’t exercise at all and I’m more of a couch potato than anything.

My friends often envy how I’m able to keep extremely slim even by eating excessively and I’m in no way glad that I’m slim. Seriously, how do I plumpen up? Am I suffering from some kind of illness?