Tsukkihina ~ secret mate

A Tsukkihina ABO fanfic... at least I think it is. This is my first fanfic... please go easy on me. Tsukki wanted to keep Hinata a secret for as long as possible because they were still young. It was Tsukki's idea to keep them secret, so why does he get so upset when he sees Hinata happy with anyone that's not him.