Marshall: But I-I think that we've figured out what's going on with you. See, after we let you go...
[flashback to Lily and Marshall's apartment:]
Lily: What is this?
Marshall: Marvin's Book of Firsts?
Lily: [gasps] Ted took Marvin to see Santa?! Oh, I am gonna take this book and I'm gonna shove it so far up his...
[present:]
Marshall: The point is, it made us remember another scrapbook you made.
[montage of Ted's "Buildings Book of Firsts" scrapbook]
Marshall: That project really was your baby, and now it's over, and you're scared to move on.
Ted: I poured my blood, sweat and tears into that building. Though, to be fair, a lot of that happened the day I accidentally fell down the elevator shaft.