Sometimes, the artificial chapter divisions added later get in the way of surveying the context of a passage. This is one such case. If you read the preceding verses, we learn what the "Wherefore" is 'therefore.'
Hebrews 5:10-14 (DRB) Called by God a high priest according to the order of Melchisedech. 11 Of whom we have much to say, and hard to be intelligibly uttered: because you are become weak to hear. 12 For whereas for the time you ought to be masters, you have need to be taught again what are the first elements of the words of God: and you are become such as have need of milk, and not of strong meat. 13 For every one that is a partaker of milk, is unskillful in the word of justice: for he is a little child. 14 But strong meat is for the perfect; for them who by custom have their senses exercised to the discerning of good and evil.
The writer wants to lead his readers on to a maturer knowledge of Christ, "not laying against" (6:1) the basics of the Christian faith, "the things of the beginning of Jesus Christ," or "the elementary things of Christ" (6:1). He says if you remain only a partaker of milk (or basics) you will never mature, but will remain a "little child," and not become "perfect" (5:14).
Obviously the author doesn't intend to throw a away baptism or repentance from evil works or the laying on of hands or the resurrection from the dead, or doctrine on hell, or faith in God—all fundamental parts of Christianity. Yet you'd be amazed how far people can go to avoid "repentance" or "baptism" from being a fundamental part of the Christian faith, among other things, turning this into a list of doctrines not actually the fundamentals of the Christian faith, but obsolete components of the Old Testament. Amazing.
"Forsaking" or "leaving now" (aphentes) the things that are for the "little children" and "those who have need of milk and not strong meat" in context refers to moving on to something else, not making what preceded obsolete and irrelevant. Clearly the strong meat is a richer understanding of what the milk already contains implicitly.