How would you feel?

Whilst cleaning I found a journal of my SO's that was wrote a couple of years ago, this book was hidden - I was not meant to find it. So naturally I looked inside, and then found out why it was hidden. It contains memories and pictures of times that my SO spent with their ex when they went travelling and lived together. It upset me because I don't understand why there would be any need to keep this, obviously they know this will upset me considering the fact that it's hidden. How would you feel? Am I wrong to be upset and angry?