When Diarmuid first met Professor Oak and Dragonite, they were startled by the bubbles. They actually thought that Lir was attacking them! As a result, Diarmuid tends to worry even when he probably doesn't need to.

"It still hurts!" Diarmuid looks up at Shinju, a serious look in his eyes, "Just because you know it might happen, doesn't mean you can be ready for when it does. The pain is still real."

It hurts Diarmuid so much to think about the fact that his brother might leave someday. He will probably end up being in the same shape--if not worse--than Shinju is now. Some pain a person can't prepare for no matter how hard they try.