I'm a woman, and my best friend is also a woman. There was never anything romantic between us, but when she got married, I felt a kind of jealousy I wasn't prepared for. Suddenly someone else knew what she ate when she was sad, heard her random midnight thoughts, and became the first person she called with good news.
I acted excited throughout the wedding, and I genuinely was happy for her. But I cried in my hotel room afterward because I knew our friendship had changed permanently. Five years later, we are still close, just differently. I've never told her that one of the happiest days of her life was also a small heartbreak for me.