Here's an incident that changed me twice:
When I was about 5 or 6, while I was sitting on my Grandpa's lap. When I told him I loved him, he asked what I thought love means. I thought about it, and I remember being squirmish as I replied "I don't know." (If I could have defined my feelings then, I would have described love as unconditional.) He told me then that love is when you would die for someone. That was a heavy statement for a 6 year old to assimilate, and I took it at face value. After he said that, I always defined my love for someone by whether or not I would die for them.
It wasn't until I was older that I realized he was referring to Christ with that statement. With that realization, I then had to re-define my standards for loving someone.
Interestingly, it's the only thing I ever remember him telling me. In that sense, the sentiment remains very precious to me.