I don't think Sherlock is 'in love' with John, but I do believe he loves him. He's probably moving out of Sherlock expected to come back and have everything go right back to normal. He is sad because it can't be his way
SPOILERS Call and response." // Also, I love how Sherlock's entire speech (the prepared part, at least) read like something straight out of canon, elegant language and blunt logic mingled with deep-running sentiment and all.