What separates a kink/fetish from a passion?

Tickling is a very common fetish... so are feet... and so are combing the two. However, just because you enjoy those things, it doesn't necessarily mean you see them as sexual - and, in fact, you could even be sexually revulsed.

ETA: Just to clarify, I'm uncertain myself. That's why I'm asking. The poll about plastic bags was what had inspired me to ask this.