To those who broke up with their partners because you “lost feelings”, what happened to cause that?

Was there a specific event or did the romance just fizzle over time? Do you regret your decision? Did you blindside them or give them hints over time? Do you feel guilty about what you did or how you feel? All commentary is appreciated.

For context, my girlfriend of 2.5 years recently dumped me out of nowhere. She said she hadn’t been feeling happy for about a month, but she gave no indication of this.