There was the myth of how the plant crowns were made.
Apollo was walking around when he saw Eros's small bow and laughed at it. He ragged about his own bow's size. Eros ominously watched Apollo walk away with an idea.
Apollo was wandering throughout the mortal world when he saw a beautiful maiden (i forgot if it was a nymph or a mortal) when Eros shot Apollo with an arrow of love. Apollo fell head over heels in love and started to try to romance her. She ran away (forgot her name D:) since she knew that Gods aren't the best people to date.
She suddenly ran onto a cliff with nowhere else to go. She called Gaea (Mother Earth) to save her. Gaea felt pity for her and turned into a tree. Apollo was heartbroken and made wonderful leaf crowns from the tree.
That's how the trend started in Ancient Greece according to myths.
Hope this helped!