Why are building and maintaining friendships so doggone hard?

I've had a couple of close friends in particular whom I have been friends with since we were young. We have kept in touch throughout the years but the relationship really don't seem to be the same. One of my (ex) best friends just had a graduation and I had no idea about it. After she told me and I replied Oh congregations! I'm so proud of you! I said I wish I had known I would've tried to make it… She just replied." thanks…."