In February, 2014, I shut down a business I'd built and run for 18 years. I took my staff to a Startup Weekend to show them what entrepreneurship looks like, and I wrote about it in Quartz: http://qz.com/173080/why-i-forced-my-staff-to-attend-startup...
Someone told me that he thought that only four of the people presenting showed any passion; I was one, and two of the others were employees of mine. (BTW, one of them - my youngest employee - won the weekend and has just recently graduated from Y-Combinator).
That got me thinking about what passion really is, and I've concluded that it isn't good enough merely to "like" something. There needs to be something about the status quo that you dislike so much that you're willing to work at addressing it, even when sometimes the work is so hard you hate every second.