In Marshall's Defense

Why Would Marshall Tell Ted to Go After Robin When He and Lily thought Barney/Robin Were Perfect in the Past?

Lots of fans were confused in "The Final Page: Part 2" when Marshall, instead of advising Ted to tell Robin that Barney's proposing to Patrice, told him to be selfish and not tell her. He seemed to believe that the chance of Ted ending up with Robin hasn't died yet. But why would he suddenly say this when he told Barney in Season 4 that he and Lily thought Barney and Robin were perfect for each other?

Ted: "Okay, why shouldn't I tell Robin?"
Marshall: "Because deep down, some crazy part of you still thinks you're gonna wind up with her, and I agree."

 

Ted and Marshall are the oldest friends out of anyone in the group. Marshall has seen Ted struggle throughout the years trying to find the love of his life; he has seen Ted and Robin date; he knows Robin has always meant more to Ted than just as a friend. They both know this. At this point, Marshall could think that Ted's closest chance to be with someone was Robin, now that Victoria was gone. Marshall wants Ted to be happy, and that's simply the duty a best friend does.

 

Marshall: "Now listen, I want my best friend to be happy, and if Robin could mean happiness for you, then dear god why help her go after another man?"
Ted: "Because she might still be in love with him. To not tell her, to not give her that chance, it's - it's selfish!"
Marshall: "So be selfish"
 

If we had to pick a relationship in the group that was the weakest, I dare say most fans would agree it's the relationship between Marshall and Robin. We don't see a lot of times where it's just the two of them, so obviously Marshall is taking his best friend's side of the issue instead. But he knows how important Robin is to Ted.

 

Ted: "Robin is my friend."
Marshall: "She's always been more than that and you know it."
 

Marshall also seems to think Ted doesn't do enough for himself - and sometimes the acts of kindness that Ted does can leave him heartbroken. Marshall might be as sick of Ted's long whirling journey of finding 'the One' as the lot of us are. So he insists the promise Ted should make:

 

Marshall: "I, Ted Mosby, promise to stop putting everyone else's interest ahead of my own and for once in my life, do what's best for me."
 

 As for why he's suddenly forgotten that he once supported Barney/Robin, he could simply like both relationships but if he had to chose who should be happy: his bestie Ted or "another man"? (as he called Barney in the second quote) Also, he's knows about the aftermath of both relationships: in the Ted/Robin case, it was because they realised they wanted different things - time could change things, and Ted and Robin have only grown closer as friends. In the Barney/Robin case, Robin's hair was falling out and Barney was fat - a bit more distressing to the eyes. Marshall went for the choice, in his opinion, that could bring more happiness to his friends.

Or some think it was because of the bet Marshall made with Lily that Ted/Robin would be end game. I agree the bet would certainly be part of the explanation, but seeing how seriously Marshall was taking the situation, I think there was more. Plus, it would be selfish on his part, as a best friend, to push Ted in Robin's direction just only because of a bet.

So I don't think what Marshall did was completely wrong. In fact, right and wrong don't make a lot of sense when it comes to BFF relationships, in my opinion, but Marshall was certainly trying to help Ted, even if it means being selfish.

(All quotes from S8X12 "The Final Page: Part 2")