The person who understands your reason can still need to explain its effect on them.

Understanding why something happened does not dissolve what it cost. Someone can accept entirely that you had to cancel, work late or move away, and still need to say that it left them alone on a difficult evening. Treating their explanation as an accusation, or as evidence that they did not really understand, closes off the only conversation that could repair anything. Both halves are true at once and both need saying.

If someone tells you how your reasonable decision affected them, listen without defending it. If you are the one hurt, say the effect plainly and leave the blame out.