I was just having this exact conversation with friends who don't play. They naturally think I'm barmy but seeing someone else having the same sort of struggle makes me feel better. So far I've only been able to bring myself to trade duplicate dragons that are standard breeding. I have a stable full of limited and event duplicates but I'm struggling to let them go. Why exactly I'm saving them, I'm not sure. I can't display them or use them in any way but thinking of trading them makes me antsy.
Thankfully after years of diligent hoarding I have lots of triplicates so I can at least let myself trade the expired dragons so long as I have one on display and one in the stable left. I've really got to work on letting them go, I know. I think that a small part of me has pie-in-the-sky dreams about one day being able to trade with my neighbors. So when I'm about to trade an expired duplicate, in the back of my mind I'm constantly wondering if one day down the line I'll regret it.
I'll give S8 credit where it is due, this game really gets it's hooks into a certain type of person. The entire reason I play is to collect the dragons but I didn't realize how obsessive I was about it until recently.