For me it was,
The death of my first dog (not that the others didn't devastate me), its just the first one just threw me into some sort of 'reality check, emotional roller coaster'. Having to have him put to sleep was so painful, from so many different angles.
The death of my Mum. I know somewhere, deep inside, you know they will die eventually. She was only 43, but I could imagine that this would still happen even if she was older. It just threw me into a 'whirlwind' of anger, and tears. A helpless situation, and a feeling of being so alone.
The birth of my first born. The overwhelming gush of love, protective instinct and the 'I would die for this little bundle' feeling .... didn't see that coming.