Let me guess. You have had trouble gaining weight whenever you have tried?  The thing is, as a skinny guy myself, I have realized gaining weight was a problem for me. So I started body building and got help through some forums and the thing they always told me "You've got to eat big to become big".  So I started counting my calories, splurging on dinner, and sometimes lunch, and I came to a realization. I don't eat nearly as much as I thought. I thought I was eating like a cow, but my body was telling me "Whoa. you've got your 2k calories, let's slow down"  Genetics plays a small part in it. Like a ridiculously small part in it. It is more of a mental state that tells you to keep eating, for comfort or whatever.  Someone with a "fast" metabolism would burn at most 200 calories more than someone with a "slow" metabolism, with no effort. It also probably helps that I'm young, active, and my calories are going towards growing.