Irrational Fears

Where does fear come from? Why are so many of our fears so completely irrational? Do they stem from events in our lives we just don't remember?

I have a completely irrational fear of being home alone. Not during the day, when the sun is out I'm fine. But at night, I hate it. I'm not ever completely alone, technically, I have Sugarplum with me. But she's 2 1/2 and goes to bed by 8. Then I'm really by myself.
I have this horrible fear that someone is going to break into the house and hurt us. I don't feel this way at all when my husband is with me. He's periodically away for a weekend and I have the worst trouble sleeping during those times. Sometimes, even before the sun sets, I just start crying - for no reason I can think of other than I miss him terribly and feel horribly anxious, isolated and lonely.
I was thinking about it tonight and realize that I have had this fear since I was a little girl. I remember at the age of 8 being home with my brother alone in the house. He was older than me so this was not really a problem - and we were never alone overnight. But if he was outside with his friends and I was inside after dark and my mother was out for whatever reason, I would start to freak out.
This was the days before cell phones so I couldn't call her and ask when she would be home. I could only sit and be afraid. So I would call my aunt or my grandmother. They would calm me down. After enough of these episodes, my mom made my brother start coming inside at sunset if she was out, and she worked hard from then on to not be out after dark.
I have no idea why being alone at night plagued me as such a small child, but it is clearly something that has carried on for these last 22 years because to this day, I hate being home alone at night. It scares me.