Developmental prosopagnosia wasn't recognised as a "thing" before the late 2000s. Before that, everybody assumed that being able to recognise someone was just something everyone, well, did. Failure to recognise a friend or relative in the street was seen as an affront, or a deliberate snub. Failure to recognise famous people in films or on the TV was something to be ridiculed. Prosopagnosiacs quickly learn coping strategies growing up in such a world, but those recognition mechanisms are significantly slower: they don't solve the problem.
It is amazing how upset even the most decent and understanding of people can get when they feel you've slighted them in some way. Having someone (you like/respect/love/etc) screaming abuse at you in the middle of the street because you failed to respond to their greeting, because you didn't recognise them and realise they were saying hello specifically to you ... is humiliating. Not knowing why this situation was happening on a regular basis - can you blame me for thinking that it was my fault? That I wasn't a reasonable, decent person? That the things being screamed in my face were accurate?
Hence my relief when discovering there was a label for my condition. Though, sadly, no cure.