Everyone needs a secret

"In the planning stage, I make sure that all my characters have secrets that will be revealed gradually during the course of the novel. Such secrets will motivate all sorts of unexpected action and furnish the surprise element that I'm trying for. Before I ever get to the writing, I examine my characters for those secrets they may be hiding, and I plan ways in which such secrets may affect the lives of other characters in the story."
- Phyllis Whitney, Guide to Fiction Writing

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Oh my. That is all kinds of awesome. I love that and I'm so glad you shared it. I now know what I'll be pondering about...
When I read that, I was like - wait, I say that all the time. Like others have said, though, I don't necessarily plan them in advance.
Definitely like that advice. It's akin to how we set up characters on an online text-based RPG I play. We give them all secrets so they have things to hand and/or reveal that will affect their lives later at their leisure.
Sometimes when I'm writing I'll realize that a character has a secret - and it's a wonderful juicy one! What's weird is if I go back to add in clues, sometimes I find I've already dropped hints, like my mind was working on it.