For John Watson, the most boring thing he can imagine is to be alone with his thoughts. He is a man of action; he needs to be doing. Invalided home, unable to work because of his injury, he feels useless, as if he is merely existing—not truly living.
Meeting Sherlock Holmes feels like being rescued. For most people, the whirlwind of that day would make it hard to breathe; for John, it is the first time he has breathed easily since returning from Afghanistan.
It’s not that John doesn’t think. It’s just that he does his best thinking while he is doing.
For Sherlock, distractions while he is trying to think are annoying. He needs quiet; he does his best thinking when alone. Though, sometimes while John sits quietly reading some frivolous magazine, he finds that thinking is still possible. If he can resist disparaging the reading material, that is.