This poem is a contrast to Infant Joy from Songs of Innocence. The child is born into a dangerous world, despite the trappings of comfort and prosperity around his bed.
Even in his first natural state of naked helplessness, the child conceals what adults would regard as an evil spirit (‘fiend’). The pressures of conformity (‘my swaddling bands’) will release this spirit in ‘struggling’ and ‘striving against’. The act of sulking on his mother’s breast suggests only a brief respite before the ‘fiend’ (properly, the child’s true individuality) finally asserts itself in adulthood.