Seriously, guys- I don't own IZ. Jhonen does.
Time traveling was sketchy business; out of all people, Dib should've known this. As soon as the rock appeared, he should've stepped back. Actually, he did. Started to walk away, even. But after a couple of steps…
It was honestly too tempting. Go back to the past, see your future- who wouldn't want that? And for Dib, future scientist, the opportunity wasn't going to get passed up. Because when I wake up thirty years from now, he told himself, to justify his actions, wouldn't I want to say that I was brave enough to pick up the rock?
The action itself isn't a heinous one; it's just picking up another random object. But when you act all stupid and say Vade, then you have a problem. And Dib, of course, just had to imagine the future. Not the past, not anything he knows; no, that's too easy to do for a first-time travel. He had to go and take himself where no mortal needed to be until then became now.
There are certain things we shouldn't see, certain images that would scar one who doesn't know what's going on. Like, for instance, when a time traveler decides to go and visit his future self, and finds said self to be lip-locked with his alien nemesis. That could scar a person just a little.
And that leads the future beings to have to do a mind-erase (which isn't easy due to the hysteric kid), and send him back to the proper time, keeping the rock with them. No explanations, just a one-way trip home. Because how would they go about telling the kid who's bent on the alien's destruction that they become allies after a disease almost wipes out the human, and the alien saved his life. Worse than that, how could they tell him about their first kiss, first date, first… well, you get the idea. They can't tell him that. So they don't.
But seriously, guys- don't time travel. It's just ends up in a wasted day.