Our ideas come from our environment. Anything and everything that we think up has been, in some way, influenced by our surroundings. Many of the greatest authors of all time have written books about their hometowns; many great novelists have also "used" friends and family members from their childhood environments to help create their characters. Although these ideas are unique, they were definitely not just conjured out of the blue through the sheer imagination of a human being. People write about what they know, and we usually know the most about our surroundings, whether they be in the past or the present.
There is nothing wrong with using your environment to help write a good story. One of my favorite stories that I ever wrote came almost directly from my surroundings. I used characters that I had encountered in everyday life, took ideas for buildings and layouts from my home, and even constructed scenarios that were unique to everyone else but that I was very familiar with. That being said, even though my ideas came from my environment in that story, I still had to shape them to fit the story I wanted to write; after all, I wanted to make a work of fiction, so I didn't just want to recount something that I had already done.
See, even though I might say that I took my ideas from my surroundings, I could only start to form my ideas from them. For example, if I'm stuck trying to create a character, I'll just think of the traits of a person I know that would fit the bill for the particular character. If I'm trying to write about a villain, I can always draw on an experience that I had with an enemy of sorts in middle school; if I need to write about a good parent, then I know to look into my own home for inspiration. The point is, even though our ideas don't always just pop out of our heads, that doesn't mean they aren't ours, because we still need to work to make them fit our tales.
Once a person has been exposed to any life at all, his or her brain will start thinking up ideas based on his or her experiences. I happen to have read a lot when I was young, and I grew up all around books. Everything I learned when I was young outside of school had to do with reading and writing, and in turn, I "formed" the idea that I wanted to be a writer when I was older. Currently, I do write a lot, so my idea of sorts was fulfilled, but what if I had been exposed to sports all the time? What about acting? I could have become a completely different person, physically, emotionally, and mentally, if I had been exposed to something different in the early stages of my life.
When we write fictional stories, a little bit of our past usually manages to sneak its way into them. For example, a writer who has been plagued by several deaths in his or her family might choose to write books and stories about family deaths because that's what the author would have the most experience with and relate to the most. It's natural for us to write about the first thing that jumps to our minds, so when someone is constantly thinking about death, destruction, or any type of family tragedy, chances are that his or her ideas will mostly fall under that category throughout his or her career in writing.
In conclusion, our ideas are formed by what is around us and what we have been exposed to; however, it is up to us to shape them into coherent sentences to construct written works. Ideas are precious, and you should always been on the lookout for ideas during your day. Who knows whether or not a conversation you had with your friend yesterday turns out to be the inspiration you need to write a great dialogue? No two people experience the exact same sequence of events throughout their lives, which is why no two people think exactly alike. Your life is unique, and it is a treasure trove of ideas ripe for the taking.