Why do we do this?

Maybe its living in a country with the highest crime rate in the world, or coming from a family who has made panic and worry a national sport, but my husband is out cycling and on hearing about three or four sirens screeching past, I immediately panicked. I phoned him - he is fine and on his way home. But, my question is this: why do we automatically expect the worst? In whatever situation, we somehow expect the worst. Maybe its being drilled into our minds: expect the worst, hope for the best. Is that really the best way to go through life, though?

Regards,
M.