One of the most quoted lines in Endgame is when Clov asks Hamm what there is at Hamm's house to keep him from leaving. Hamm responds, "The dialogue" (1.582). Dialogue in the play is the way that the characters keep up their hope, the way that they keep from giving up. Hamm is the one who most often pushes the language along; at times, he chides Clov for not keeping up with him. The result of this situation is that the language they use is not quite natural. It is theatrical; the characters are speaking because they feel that they must speak, not just because they feel like it. They are performing, not just for us, but for themselves, reminding themselves that they are still alive and capable of continuing.
The reason that the characters treat others so poorly is that their short staccato dialogue leaves no room for compassion. Their dialogue is not genuine interaction; it is simply convenient, as a way of keeping themselves alive.
Language has taken on a sensory quality for Hamm, who can't see or move. He is as reliant on language as most people are on vision, and the reason that he forces Clov to keep speaking with him is to remind him that there is such a thing as an outside world.