John has a depression linked eating disorder. In the opening of Pink, after John wakes up, his breakfast consists of an apple that he doesn’t eat. This is very common in PTSD, especially those who have suicidal thoughts and tendencies, which we know John does. It’s not that they are actively starving themselves, it’s that they just don’t see the point eating, as an effort to stay alive.
When he meets Sherlock, John eats dinner as if he is starving. He digs into his food, talks with his mouth full, as if it is the first full meal he has had in months. Which is entirely likely, since John has been home for several months at that point. Again, this is very typical of people who have been suffering this type of eating disorder, and find that they are no longer as painfully depressed.
Sherlock even goes so far as to point out that after moving in together, John puts on an average of a pound a week in weight. John brushes it off as being normal. He doesn’t deny it, he just points out that he is eating more than one meal a day. This implies that he wasn’t eating this often before he limped into Sherlock’s life.
This makes Sherlock’s insistence that John eat even more powerful. He goes so far as to halt an investigation on more than one occasion, to make sure John gets a meal into him. A well-fed John is a happy John, not because he is full, but because it proves that he is happy enough to actually eat.