Also, in almost every religion save a handful (like the Germanic/Norse mythologies and the Egyptian ideas about the afterlife) you lose your body completely, and so you also lose the human passions and emotions that distract (eastern concept) or tempt (western concept) us. Eternity for our freed or resurrected souls would be an entirely different experience than for our embodied souls.
And there is the fact that our human brain can't really understand the concept of eternity, only use it. So whatever we would imagine eternity to be, it's only a vague estimate at characteristics of it, not an understanding of the thing itself. Whether we value those characteristics as positive or negative doesn't depend on eternity itself, but on our associations with those characteristics. Just like all our individual concepts of dragons and gods are approximations, so are our concepts of eternity or nothingness.