Around six days before Passover, Jesus revisited Bethany (where he rose Lazarus from the dead). At the dinner table, Martha served while Lazarus sat with Jesus. Additionally, Mary used spikenard oil to anoint Jesus and even wiped His feat with her hair.
From a later comment, the spikenard oil is rather expensive. But even then, Mary used her valuable to anoint Jesus. Anointing involves rubbing oil but also symbolized choosing someone. Thus, with the anointing, Mary chose Jesus to indicate that He is of high value to her. It was also likely done out of thankfulness for Jesus raising Lazarus from the dead.