In using milk as a metaphor in I Peter 2:2, Peter is in no way chiding people as Paul does in Hebrews 5:12-14. The former uses milk simply as a nourishing food because his emphasis is on desire, not depth. Paul uses milk as a metaphor for elementary because he wants to shock the Hebrews into comprehending how far they had slipped from their former state of conversion.
Paul also uses milk as a metaphor for weak or elementary in I Corinthians 3:1-2: "And I, brethren, could not speak to you as to spiritual people but as to carnal, as to babes in Christ. I fed you with milk and not with solid food; for until now you were not able to receive it, and even now you are still not able." Paul judges the Corinthians as weak based upon their behaviors and attitudes, which reflected no spiritual progress. So he "fed" these immature Christians elementary knowledge because things of greater depth would have gone unappreciated, misunderstood, and unused. These references directly tie spiritual diet to growth in understanding, behavior, and attitude.
Paul's milk metaphors are scathing put-downs! Undoubtedly, he seriously hurt the feelings of many in the congregation, yet he is free and clear before God of any charge of offense. He does not question their conversion, but he certainly rebukes their lack of growth. He rightly judges that they need to have their feelings hurt so they could salvage what remained of their conversion.
In I Corinthians 3, the embarrassing immaturity that required him to feed the people like babies also produced strife and factions in the congregation, proving that the people were far more carnal than converted. The Hebrews account is more complex: The people had once been more mature but had regressed. It is a situation vaguely similar to elderly people becoming afflicted with dementia, except that faith, love, character, conduct, and attitude were being lost rather than mental faculties. This resulted in the people drifting aimlessly.
An additional insight regarding an insufficient spiritual diet appears in the next chapter. Paul tells them that their problems are directly related to being lazy. Dull in the phrase "dull of hearing" in Hebrews 5:11 is more closely related to "sluggish" or "slothful." It is translated as such in Hebrews 6:12, ". . . that you do not become sluggish, but imitate those who through faith and patience inherit the promises."
Paul charges them with being lazy listeners; they are not putting forth the effort to meditate and apply what is taught them. They are, at best, merely accepting. That they are not using what they hear is proof enough for Paul to understand that they are not thinking through the seriousness or the practical applications of the teachings. In other words, they are not assimilating what they hear, and the result is a lack of faith and a consequent faithlessness. His rebuke is far more serious than the one in I Corinthians 3 because these people are older in the faith. They have frittered away a large amount of time that would have been far better spent on spiritual growth.
Paul attempts to shame and shock them into realizing how far they had slipped by calling these grown people—some of them undoubtedly elderly—infants. He goes so far as to tell them that they are unacquainted with and unskilled in the teaching on righteousness. In other words, he attributes to them the one particular trait of infants: that they do not understand the difference between right and wrong, a characteristic that defines immaturity. A parent must instruct and chasten a child until it understands.
The Bible provides ample evidence that a poor spiritual diet results in a spiritually weak and diseased person, just as a poor physical diet works to erode and eventually destroy a person's physical vitality. Similarly, we can see that a person can be in good spiritual health but lose it through laziness or another form of neglect. Just as a mature adult needs good, solid nourishment to maintain his vitality and remain free of disease, the spiritual parallel follows. For one to grow to spiritual maturity and vitality, a mature Christian needs solid, spiritual nourishment, assimilated and actively applied, to continue growing and prevent regressing, as opposed to the Hebrews sluggish spiritual deterioration.