By far one of my favourite manga and anime series of all time. It's an adorable and gorgeous story of two hurt people finding love in both themselves and the many friendships they grow. If you want a tooth-rottingly sweet romance between a 'monster' and a human this is more or less the pinnacle.
This is not really an angsty romance anime though, so if you're looking for some sweet sweet romantic pain this won't be your best choice, but I still think you'd enjoy it for the many other reasons for which this anime is great.
If you liked series like Akatsuki No Yona (Yona of the Dawn) or Shirayuki - Akagami No Shirayukihime! (Snow White with the Red Hair) this is a must-watch.