Maybe unfair to dump this much sentiment onto his trade, but it's not one player, it's an entire era that's leaving us.
I was in college and into my young adult life when that Bruins team emerged from the cellar and began to be a real powerhouse from 2009 onward. Bruins were just fun, and Marchand and his antics became a major part of that. No matter how many times he crossed the line, I always defended him out of fan tribalism. But he matured, and flourished into a pretty elite winger in the second half of the Bruins' run.
That era was just so fun - Lucic, Thornton, Horton, Recchi, Chara, Krejci, Bergy, Seidenberg, Thomas, Rask, Krug, etc. And Marchand was the last one standing, so it closes the book on that era and there's a sadness there. Like closing the book on my youth at the same time.
It was time though. Sweeney did the right thing. You have to rip the band-aid off to move on at some point and start a new era.