"It's not legendary unless your friends are there to see it".
I was thinking about the last few episodes and how Barney should have been stoked to travel the world with Robin. Think of all the legendary things the Barnacle could do around the world, with a legitimate excuse to be there (My wife is on assignment with WWN). Why wasn't he happy. And then I remembered him telling those 2 young men this line and that's when it hit me. He's miserable without his friends and these adventures are no longer legendary because they aren't there to share it with.
And the repeated "It's great" wasn't him trying to convince Ted he was happy with Robin when he really wasn't. It was him trying to convince Ted and himself that he wasn't miserable being away from the people that made his life legendary.