On a number of occasions, Jesus singled out Peter, James, and John. When He raised the daughter of Jairus from the dead, He brought along Peter, James, and John (see Mark 5:37). Peter, James, and John exclusively witnessed the transfiguration of Jesus and His conversation with Moses and Elijah (see Matthew 17:1–3). And in the Garden of Gethsemane, Jesus took the three with him and asked them to wait and watch and pray.
Did Jesus call them aside because they were more spiritual than the others and He was rewarding them for their godliness? That could be. But I have another theory. I think they may have needed special attention.
Could it be that Jesus took these three aside to keep His eye on them? Don't forget that James and John were known as the Sons of Thunder. We catch a glimpse of why they earned that nickname when a Samaritan village was not hospitable to Jesus, and James and John asked, "Lord, do You want us to command fire to come down from Heaven and consume them, just as Elijah did?" (Luke 9:54). As for Peter, well, his faux pas are legendary.
I think Jesus was preparing Peter, James, and John for what was ahead, because each of them would have a unique encounter with death. Peter and James were martyred, while John survived an attempt on his life.
Sometimes there are trying circumstances in our lives that don't make sense to us. We wonder why God is allowing us to go through them. But God is preparing us for what is ahead. Everything we go through in life is preparation for what will follow. God will always give us what we need, when we need it. Not before—and never after—but just when we need it.