Eternity is the last of the cosmic beings like Infinity, Oblivion, and Death. He's made up of everything living in the universe, with the exception of the Living Tribunal and The One Above All. You better believe that Eternity felt it when Thanos snapped his fingers. That must have taken Eternity's breath away at the very least, although I'm not sure how much Eternity actually cares about life. If you are life, then you better understand that life is fleeting or else you'd be one sad dude.
Think about this, if Eternity is every living thing, he is also all the annoying people in the world. Do you think he sees someone like Guy Fieri and just wants to hit up Death, "hey, bro. Can you take out this Guy Fieri guy. I don't want him in me anymore."