Why do our insecurities and fears come out in our dreams?

Is this true with everyone, or just something I'm noticing of late due to all the drugs I'm on?  Last night I remember 2 very distinct dreams.  So vivid, I can't shake them (lately this has been how I spend part of every day).
In dream one, the zombies were after us.  BD, Frank, and I were living in a house where there were already some zombies, and BD found a way to cure the zombies.  He took vegetables, similar looking to celery, broke them open, and an oily substance poured out of them.  We covered ourselves in this oil, and it protected us from zombie bites or infections.  It also healed the zombies.  Who would have guessed vegetable oil?  It was a hectic dream with people living in small quarters, sharing rooms, lots of mad crazy fight for your life kinda stuff.