When the person will not see you.
When the person will not talk to you on the phone or similar.
When the person is dead.
When you have completely lost touch with the person and need to approach them in stages.
When they are much older, more senior, more important, or more strapped for time.
When you need to approach the person with particular tact and consideration.
When the subject matter is particularly sensitive.
When the harm involved considerable embarrassment or humiliation of the other person.
When the amend needs to be worded very carefully.
When there is a lot of material to get out onto the table.
When there is a risk the other person will misconstrue what is said.
When there is risk the other person will prevent you from completing an orally delivered amend.
When there is a risk of the conversation becoming derailed.
When there is a risk of other matters eclipsing the amend.
When there is a risk of a romantic or sexual flare-up.
When the harm consisted in violence, stalking, nuisance, interference, or intrusion.
When they have said they do not want to see you or speak to you.
When they terminated the relationship in the first place.
Letters can often open the door to a phone call and / or a face-to-face meeting.
Indeed, a more immediate expression of regret and follow-up should always be offered.
The job is to get the job done in the best way taking into account the other person and approaching them the way we would want to be approached if the tables were turned.