My thoughts.
I don't remember how I was introduced to Diancie. I just remember falling in love with it when someone in a Secret Base in my Alpha Sapphire game had one on their team. I even made sure to set their base as a favorite so that I wouldn't lose him and his beautiful Diancie. Some time later Nintendo did an event that allowed us to download a Diancie via Nintendo Network for a limited time so obviously I jumped on it as soon as I could. I am so happy I did because Diancie is amazing.
Now, usually I'm not overly keen on using Legendary or Mythical Pokémon. I think mostly it's just because of the Legendary status or something. I'm not even sure anymore. My mind is just kinda blank on this. In Alpha Sapphire I ended up having a Latias on my team for the entire game so I think this changed my mind on this. When I got Diancie I'm sure I never once put it in the PC. I had to keep it with me all the time. Of course, it was post game when I got Diancie so I suppose this didn't matter too much because I was using Kyogre and Lugia too. I don't know. I'm just rambling at this point. Sorry!
So what is it about Diancie that appeals to me so much? I know it's not because of it's Legendary/Mythical status. I think mostly it's just how it looks. Diancie is just so darn pretty looking. I'm not one who is overly fond of gems and the like so I don't think that plays any part in it. Maybe. Okay! I like Diancie because it strikes me as beautiful, cute, and awesome. It kicks butt in battle and is just all around fun to use. With the introduction of the Fairy-type it has since become my new favorite type so I like to think that had some part in it too.