I noticed that their attitude changed after I hadn't been able to kill myself after a year or so of getting my license, and the few relatives that did go up with me came back (in one piece) with good stories.
I guess flying a light aircraft, even as a passenger, is just so wildly out of their comfort zone that for some the step is simply too big.
Oh, and maybe they've always seen pilots as the supreme skygods, and can't believe you're one of them now. What surprises people most, I noticed, is not the fact that I can fly an airplane, but that I'm allowed to do so without supervision.