The why is actually an easy enough question to answer. Cute makes people happy and it makes them smile. There is nothing Diarmuid loves more than making people happy, so it only makes sense for him to be as cute as possible.

It also helps he seems to be a natural at it...

Diarmuid stands up and bows his head to Rhea, thanking her for the compliment, and then adds his two cents into Yu's answer, "He said that you were the only one we could trust and that the scientists weren't being honest with us!"