I've got a couple.
In fifth grade. I'd been bullied for years. Bullying at the level which I cannot even begin to discuss. I was psychologically beaten down and couldn't even conceptualize defending myself. Then they started picking on my brother. I snapped. I spent the whole year beating up every person who tried to bully me, and by the end none of did. I still didn't have any friends, but I had myself. I found myself at the age of 10, and I wasn't going to let anyone take me from myself ever again.
The second, at the age of 17, was meeting and then dating my boyfriend. He was the only person I had truly connected with in years. I wasn't a particularly social person, what with all the emotional scarring during my formative years, and up until then I was seriously starved for genuine affection from my peers, and he gave me that, and so much more. He has lasted five years so far, cause I like him and all.
The third, was in the middle of freshman year at college. I was drinking a cup of chamomile tea, and suddenly, my experiences up to that point clicked, and I felt like I could see things through my own eyes. It's difficult to explain, but it completely shaped my emotional and social life since then. I made friends, real friends. I hadn't had that before.
The fourth, is my younger brother's death. My heart broke in ways I didn't know were possible. I hurt in ways I had never known, and I have a hole in my world that won't close. That same year seven other people I knew died, some closer to me than others. That type of thing... changes everything. How you relate to your emotions, how you deal with other people's emotions, how you think about humanity as a whole.