I propose a theory. Mycroft told John something very important here… You see, previously John stated that “He (Sherlock) is not like this, he doesn’t feel this way”. And I think that Mycroft wanted to tell the doctor that Sherlock can feel and that he can be in a romantic relationship. When did he tell that? When he said that his brother has the brain of a philosopher or a scientist, yet he elects to be a detective. Think about it for a second. Sherlock could be either a scientist or a philosopher… He could, because he’s capable of being one, his brain is “wired” that way. And yet, he chose to be a detective. (A discipline that requires both, scientific methods to describe the crime itself and philosophy to understand motives). So he elects to use everything that his brain is capable of doing; being a detective.
The next part: what might we deduce about his heart? Well, going by his brain, we can link these two and we get a clear picture. Remember that Mycroft said this after John’s comment on Sherlock’s capability to do feelings… Mycroft gave John a hint that Sherlock is capable of both aspects of love (friendship - spiritual as in philosophy, and passion - physical as in science). He told John that Sherlock can be both a friend and a lover and that above all he chose John. He chose him, and (as in being a detective) he combines both things in this relationship.