How do you deal with grief you don't understand?

Today, my mother told me that a man I had met only once several years ago died last year. He came to visit us when we first moved to CT and we spent a few days together. We had a great time and he declared himself to be my honorary grandfather because everyone needs two grandfathers. over the years I haven’t spoken to him but I have thought about him a lot. now that I know he’s dead, I am very sad. I’ve been crying for the better part of an hour and I do not understand why. Can the collective offer me any advice?