1.
Jesus’ humble unwillingness to pronounce himself “good” while he was still in mortality, his mission not yet completed, is mirrored in the distinction between his statement recorded in Matthew 5:48, spoken before his crucifixion, and the quotation from him given in 3 Nephi 12:48, which was spoken after his resurrection and ascension.
2.
It’s not clear from this story — and I think it very unlikely — that Jesus expects all of his followers to give away all of their possessions and become itinerant preachers. But precisely that, I suspect, is what this young man needed, and where he could have made his greatest contribution. In other words, Jesus’ words to him were exactly calibrated to his personal case.