In the very pragmatic sense, I believe people value their existence on what they did, and there are other parties to judge. Sure, the degrees of others' judgement and how it affects us differ between one another, but it affects you more or less. In which case, for people to disregard their existence or its value was pretty much a devastating thing by itself.
Going meta...tho.... well, everyone had said things more beyond my small comprehension. I agreed with 'sensory', 'thoughts', and mostly memories being a proof of our existence.
However, the (hopefully) theoretical situation of me, you, each of us being only a figment of someone's imagination or computation do makes sense. I remember playing a PS2 game, Star Ocean : Till the End of Time, which plays around with this.
I don't know the answer, and I don't think I'm seeking it at the moment. Our existence...is there. Here. Even if some time after this, I woke up ala The Matrix, or a-la Star Ocean, that doesn't make our past experience, our memories, any less true than what it's supposed to be.
Or there's this metaphysical idea of a 'heart' or 'soul'... but I'm not going there. This is hurting my head more than I'm willing to be (wait; why am I posting here, then? No idea..)