Flowers are an expression of the moment they exist in. 

A physical extension of a feeling. They say "I needed to show that I cared about this specific time in our lives!" 

"I'm so proud of you for graduating!"

"Will you marry me?"

"I'm so sorry your mother passed."

...and as sure as those moments will pass, so will the flowers.

Flowers and theatre are similar that way. Live! Communal! Present! So much of the beauty lives in the fact that the moment will never exist again. 

Precious. Worthwhile. Ephemeral.