The light of somebody’s life
Definition 1 of 1
Pro Tip 1/3Emotional Value
The phrase expresses strong emotional attachment and joy brought by someone special. 
Ever since she adopted her puppy, it has been the light of her life.
Pro Tip 2/3Metaphoric Glow
The term 'light' is used metaphorically to suggest brightness and positivity that a loved one adds. 
My grandfather is the light of my life; he always knows how to cheer me up.
Pro Tip 3/3Not Literal
Remember, this is not about physical light. It's about someone's importance and the happiness they bring. 
His mentor had been the light of his life, guiding him through tough times.