"Yeah, now they're putting me down, too. Its not fun, is it?"
I always felt that Mycroft reminding Sherlock about Redbeard was his way of telling him, "Remember that time you loved something and it was taken from you? Don't let it happen again." Mycroft knows that Sherlock loves Mary and especially John, and is telling him not to get involved because they will be focusing more on each other now with no time for Sherlock.