What causes the element of surprise when the child comes on the scene of 'adult reconciling'?

As Larkin is absorbed in the resonant humming of the thrush, he transcends present to his boring childhood, which he feels is best forgotten. The poet is transformed into a child. He feels happy like a child who feels happy just by watching elders reconciling with each other. The child comprehends nothing yet smiles just because the adults are happy. This might appear surprising however, if one may look more closely, the scene reflects the innocence of a child. Probably the poet has tried to make a point that our happiness lies in other's happiness. The whole thought makes Larkin happy and he wonders about the mystiques of universe and human life.