Why do people you trust always let you down ?

I think it's because it's hard to know everything about someone. Even when you're doing your best to make good choices about who to be close to, sometimes people who have come across as nice turn out not to be. It took a long time for me to figure out how to make those choices. I spent so much time being abused when I was young that I started clinging to anyone who showed the slightest bit of kindness to me, and often those people later turned out to have hidden agendas or not be the person they had presented themself as. It's hard to know.