I'm not trying to be pretentious or anything like that, but my first girlfriend actually managed to cause a zeitgeist shift in my thinking (zeitgeist in this sense meaning 'a particular world view which is prevalent at a particular period'). Suffice to say my world was turned upside down after I realized what had happened.

Other than that, nothing comes to mind, as it was pretty steady enough that I didn't notice any change until it was too late.