Castroneves was chasing a record-tying fourth Indy win, and a mind-boggling 16th for team owner Roger Penske.
Marco, meanwhile, was battling the legendary "Andretti curse" at a track that has generally produced more maelstrom than milk-drinking for his family.
Unless you're Rick Mears, Al Unser or A.J. Foyt, you can't assess whether a fourth win is better than a first. Andretti, the 27-year-old son of his team owner Michael and grandson of auto racing legend Mario, is still chasing that elusive first one.
Maybe that's why Marco wasn't especially chatty after his fifth near miss in the race that matters the most to him and his family. Andretti has finished fourth or better in five of his nine Indianapolis starts.
But never first.
"We were close, but we never really dominated," Andretti said through a team spokesperson. "Every time we got to the front, we got shuffled back.
"I think we did what we could, but congrats to Ryan [teammate and race winner Hunter-Reay]."
Andretti basically finished third on a day when he had a third-place car. He got the maximum out of his Dallara-Honda package, but on this beautiful sunny day, it wasn't quite fast enough to beat Hunter-Reay and Castroneves.
"Marco gave it a heck of a shot," said Michael Andretti. "Unfortunately his car just wasn't quick enough there in the end. He drove a really good race, as it seems he always does here. He's one of the best drivers I've ever seen around this place."
Marco certainly gave it his all despite appearing to have too much downforce built into his car to allow it to compete for the win. He raced wheel-to-to wheel with Castroneves and Hunter-Reay, having several close calls.
"I think if it wasn't for the Indy 500, I would be pretty mad at Ryan," Marco Andretti said. "But it is for the Indy 500. He's up there and I'm not, so what are you going to say? I think this is as competitive as IndyCar has ever been, I don't care what anybody has to say."
Michael Andretti was lucky in the sense that he had two of the top three cars competing for the win. With Carlos Munoz and Kurt Busch also in the mix (they ultimately finished fourth and sixth, respectively) Andretti was looking good as long as Castroneves didn't prevail.
But Michael admitted that he felt conflicting emotions in the closing laps when he realized his son was probably going to fall agonizingly short yet again.
"I'm secretly watching [Marco], saying, 'C'mon, get up there! If you can pass him, do it!'" Michael said. "But I saw his car didn't have the speed. I think they were both really good, but I think [Hunter-Reay] was a little more trimmed, and had a little more speed. I knew at that point if we were going to win it, it would most likely be with Ryan.
"As a dad, you want him to be up here -- I can't lie," Michael Andretti added. "It would have been so special. It's special having Ryan here. When it's your kid, it's a different thing. It's a weird feeling because I really was disappointed for him. I know you only get that many shots.