How do you stay "positive" when not getting hired after doing so many interviews?

I hate that feeling of rejection after not landing a job, especially when you got as far as having an interview. I went for an interview at the beginning of this week, and I think it went well. It was one of those interviews that felt like a casual conversation; laughing, joking, etc. For this job, I have years of experience, so they wouldn't have to waste much time training me on a lot of stuff. But something is nagging me that I may not get this job, too.

I'm starting to feel defeated. So, how does one stay positive when getting rejected after interviews?