Maybe it's because the grandma just knows where her granddaughter should be placed,I mean,Khun Chai and Jaew's status in life are like heaven and earth,which means they can't be together for real.Heaven will never bow down to earth and the earth will never reach the sky.But if the Cinderella story would come to life,that's the time they can build a love story.
she's afraid that her granddaughter will fall in love with him and from his personality she's scared that he might get sick of her and break her heart... I think that's what it said in the spoiler.