Kyouya, who immerses himself in numbers and whose life is devoted to money and success and whose social attitude has one setting, that being can I profit from this person, does not understand that the best things in life are free.
Tamaki likes being rich. He likes servants and not having to do things himself if he doesn't want to, he likes getting what he wants and he likes fancy things and the fact that he'll never actually have to learn how to drive. But he does understand that you can't buy some things, and those things tend to be best of all.
Days when the sun shines and the birds sing, and days when it rains that are perfect for sitting near a fire and looking out the window. Laughing. Singing, even if you're no good at it. Music, fresh air, the absolute knowledge that nothing can go wrong when you have a really good plan. Feeling safe. Waking up on a Saturday morning and not having to get out of bed. Being alive and being happy.
Hope. Friendship. Love.
Kyouya doesn't understand these things; he understands money, and numbers, and facts. Not simple, wonderful things that don't cost anything at all.
Someday, Tamaki will make him understand.