This is probably one of the most common recommendations to writers, and it is true. If you're writing about something you're passionate about, you're more likely to stick with the project in the long run.
Is there a cause in your life in which you invest an immense amount of energy? Or was there an event that changed the way you live? Consider using these as a jumping off point for your writing. Develop a plot around a similar event or use the cause to develop a character sketch.
Consider the recent influx of historical fiction in the book market. Most of these writers (unless they are writing about recent history) were not alive during the time periods in which their characters live.
Research helps to make their characters, plots and settings authentic and accurate. Research a time period you're interested in or that you want to know more about. Consider exploring your family's history and find a place to start from there.
Take time to sit in a public place, whether it's a mall, park, outdoor café or bookstore, and watch how people interact. Take notes about their physical behaviors and conversations.
Imagine a story about someone you come across. What characteristics does she portray? How does her body language let you know this? What problems does she face? What job does she hold? Create a narrative around the details you observe.
You never know when inspiration will strike or a story will begin. Write it down before you forget. Review what you write down on a regular basis and see where you could add or combine threads to create an interesting book.
It is natural for a writer to fall in love with his book, but he has to remember to be objective. Consider having a second party read your writing, and make sure it's someone you trust.
Keep in mind that criticism and feedback may end up killing some of your favorite parts. If so, take a step back. Are those parts really helping the story or would they be better used elsewhere?