They didn't understand the saying which he spoke to them.
"Jesus’ reply, though gentle in manner, suggests the establishment of a break between himself and his parents, although this will be modified in Luke 2:51. There is thus a tension between the necessity felt by Jesus to enter into closer relationship with his Father and the obedience which he continued to render to his parents." [Note: Idem, The Gospel . . ., p. 128.] 