Well, this is an interesting question, with a variety of interesting answers given so far. My reasons are a little multi-faceted.
We garden, hunt and fish for the majority of our food. Not because we have to financially, but because it is a life we enjoy, i don't have to worry about e. coli or other such nonsense, and game is much healthier for you. My hubby has some major heart issues on his side of the family. I want to keep him healthy. We have been eating game for so long that eating beef makes us ill because of the amount of grease in it.
So part of raising chickens has to do with expanding upon feeding our family.
Toss in this mix the fact that my children (with the exception of my youngest) are grown and gone. I have been a parent for my entire adult life, so the empty nest left me seriously not knowing what to do with myself. I am not a person who has a lot of friends. I have tons of acquaintances, but not many people who I am truly close to. The ones that I am, are all busy with their own lives and really do not have any free time for getting together and doing things. My Mom passed away in 2009, so I lost the absolute closest person to me at the same time that my children were spreading their wings and flying. My hubby and I are extremely close, but he wasn't ready to have ALL of my attention. I expanded gardening to include composting, and it is so "passive" that I still had too much time on my hands. You can only do so much quilting, crocheting, gardening, canning, etc. until you get a little tired of spending your time that way... I was BORED!
I have always been one who is fascinated by the way people lived without all the amenities we have at our disposal today. I have used a washboard to do my laundry at the creek, hung my clothes on the line, baked our bread, used some of the tools that Native Americans used to hoe my garden, etc.
Raising chickens just seemed like a good fit. I love them. They are each unique, have their own "personality", provide me with food, and give me something to take care of. A side benefit that I never expected was BYC. I haven't made any specific friends on here that I talk to all the time, but I have met a variety of helpful people, and have read some very interesting conversations.