What if one had a drug or alcohol addiction that got worse over time?
It could start out as small issues and build into bigger ones that may or may not be unforgivable.
Then the one with the problem gets sober. It may or may not be the time that really takes, but if B had the addiction, A would have probably heard the “I’m clean” line a million times. Person B could be the best person ever now, but person A who would still love them, would have a hard time putting all of that behind them without being leery to a point.