The first time i noticed this it stung a little because you start thinking that your the cause and have to fix it but after a while you'll notice it's just the way of things.
It comes with the territory of being a YouTuber. We all lose one or two every now and then mainly because these subs tend to be dead channels that YouTube removes.
Losing subscribers is part of being a YouTuber. Sometimes, it has nothing to do with your content. People simply unsubscribe because they have found a different interest.
only one sub? don't worry. it's only one guy who didn't like your video's anymore. if you create good content then you wil get that one sub back later and even more subs wil be added. besides we can always help you and support you if you need it
The more you grow the more you'll get in the process of gaining and also loosing subs.
I gain but also lose a sub a day at least but stay stable and growing so that is important!
This is bound to happen no matter how large or small a YouTube channel you have. Don't let it get you down. Just keep on creating and don't worry about numbers. Good luck!