What does the poet's smile in the poem 'My mother at Sixty Six' show?

What does the poet's smile in the poem 'My mother at Sixty Six' show?

 The poet feels the old familiar fear of separation while bidding goodbye to her father. She fears the ultimate fate of human beings. However, she is her ageing mother. So, she regains self-control to compose herself and tries to look normal. She tries to hide her ache and fear by smiling naturally.