Babies cry. Maybe it helps to know what an infant's 'normal" crying pattern is. Fuss/cry durations peak in the first 2 months (peaking average: 6 weeks), are highest in evenings, and decrease approximately 50% by 12 weeks of age. So, the first two months are the worst. Also, not all infants are alike; some are very compliant, some are very persistent, and there's everything in between.
In 1972, two researchers at Johns Hopkins, Silvia Bell and Mary Salter Ainsworth, wrote a seminal parer on infant crying ("Infant crying and maternal responsiveness") that challenged the (predominantly male espoused) idea that quick response to an infant's cries led to "spoiled babies". Further research has supported their conclusion that prompt maternal response to infant crying led to less crying and better language and communication development by the end of the first year. Form the abstract:
...consistency and promptness of maternal response is associated with decline in frequency and duration of infant crying. By the end of the first year individual differences in crying reflect the history of maternal responsiveness rather than constitutional differences in infant irritability. Close physical contact is the most frequent maternal intervention and the most effective in terminating crying. Nevertheless, maternal effectiveness in terminating crying was found to be less powerful than promptness of response in reducing crying in subsequent months. Evidence suggests that whereas crying is expressive at first, it can later be a mode of communication directed specifically toward the mother. The development of non-crying modes of communication, as well as a decline in crying, is associated with maternal responsiveness to infant signals. The findings are discussed in an evolutionary context, and with reference to the popular belief that to respond to his cries "spoils" a baby.
How, then, does this myth that letting a baby cry is good for the infant? It seems people confuse cause and effect: they think that a quick maternal response (the "cause" rather than the effect") to a baby's cries trains the baby to cry more often. (Of course a baby will cry again. Babies cry. That's how they communicate. But they do not cry more!)
There have been debates in the literature as to exactly what "secure attachment" and other variables mean, but in general, the sensitivity to maternal response to crying and infant contentedness has a positive correlation.
A 2009 study agreed:
This study examined associations between mother–infant nighttime interactions and mother–infant attachment when infants were 12 months old. ...Mothers of securely attached infants had nighttime interactions that were generally more consistent, sensitive and responsive than those of insecurely attached infants. Specifically, in secure dyads [mother-infant pairs], mothers generally picked up and soothed infants when they fussed or cried after an awakening.
Whatever the belief, it is clear that
human infant crying evolved as a primarily acoustic, graded signal, that it is a fairly reliable, if imperfect, indicator of need for parental care and that its primary function is to promote parental care-giving.
Some pediatricians see evidence that if a baby's crying is ignored, the more compliant baby gives up, stops signaling, becoming withdrawn once realizing that crying is not worthwhile, and (maybe?) concluding that he is not worthwhile. The baby loses the motivation to communicate with his parents, and the parents miss out on opportunities to get to know their baby. The persistent infant (highest-need babies) don't give up, instead crying louder and escalating, making his cries more and more disturbing. This tends to upset parents, who see it as a power struggle.
Dr. Sears recommends a middle approach:
A quick response when baby is young and falls apart easily or when the cry makes it clear there is real danger; a slower response when the baby is older and begins to learn how to settle disturbances on his own.
However it is handled, I agree that is it not possible to spoil a young baby. When a baby is older and can be taught to self-soothe, it's more appropriate to respond to different cries differently.
Infant crying and maternal responsiveness
Nighttime maternal responsiveness and infant attachment at one year
Maternal sensitivity behavior and infant crying, fussing and contented behavior: The effects of mother's experienced social support
An ethological analysis of human infant crying: Answering Tinbergen's four questions
Why do babies cry?