The actor, who plays Blaine in the Fox musical comedy, said that the idea of Rachel (Lea Michele), Kurt (Chris Colfer) and Finn (Cory Monteith) leaving the school is about "growing" their characters.
"This goes back to the beginning of the season where everyone was in a huff about people graduating and leaving, but I don't think it's a matter of saying goodbye so much as it is about moving on," he told TV Guide.
"It's about having a departure point and having grown into another person from the person you were when you started high school. [Showrunner] Ryan Murphy is a big believer in the idea that high school is a machine where you come in as one person and you exit another.
"This last season is more about growing and learning new things and being a new person and bringing that to the next step in your life. I don't think it's a hard-fact farewell per se as it is cathartic change and the beginning of a new chapter."
Of the three confirmed to be leaving New Directions behind, Criss said that he is most "curious" about what the future holds for Finn.
"Whereas Kurt and Rachel have the NYADA thing, which is an exciting next step, I like that Finn's future is a little bit more uncertain," he said. "As a fan of the show, I'm curious to see what happens for him."
Colfer recently said that the concept for Glee's fourth season is "something revolutionary".
Glee airs Tuesdays at 8/7c on Fox. UK viewers can see it Thursdays at 9pm on Sky1.
> 'Glee': Fox co-chair teases "powerful" finale, "exciting" season four