Any attempt to impute motive to the Almighty God, or to restrict his range of action by defining a norm is at best error prone and possibly dangerous. Instead we should look for guidance upon how we should make ourselves suitable for His service.
The pride of man requires him to try to understand and analyse everything, even God. Yet we are warned (Is 55:8-9) that our thought patterns do not naturally follow the pattern of God's and that he works upon a higher plane than the one we do. In fact we are specifically told in Rom 9:20 that we should not attempt to cross question God with regard to the issue of which people are fitted for which sphere of service. The next verse shows that this is an issue of sovereign will. The metaphor is very illuminating, God is a potter, we are vessels made from clay, he can make vessels for whichever purpose he deigns at the point he throws the pot.
Any attempt to deduce a pattern from biblical examples can easily lead one to believe that God picks the person least suited to the role he has planned! When God wanted someone to be the father of many nations he picked Abraham, a man whose body was counted as dead and whose wife was barren (Rom 4:19). When God needed a confident, charismatic leader to bring the Israelites out of bondage he picked a meek man (Nm 12:3) with a speech impediment (Ex 4:10) that had already been rejected once by the Jews (Ex 2:14)! When God needed a man to raise the spirits and lead an army he picked the least member of a poor family (Jdg 6:15). This pattern didn't change in the New Testament either; when God wanted an apostle to the gentiles he picked Paul, a Jewish Zealot (Php 3:5-6).
The other question we have to ask ourselves very carefully is "What exactly defines success?" Under the old covenant there was generally a correlation between prosperity on earth and righteousness (consider Ps 1:3), although there were some notable exceptions and anomalies (e.g. Job). In the New covenant measurement using a human scale is impossible. Consider Paul again, beatings, imprisonment, near drowning, stoning, robbery, desertion and the list goes on (eg 2 Co 11:23-28). Even our Lord himself gained victory through suffering a horrendous death.
One may be tempted to deduce from the above that it is impossible to influence the will of God and that therefore we can simply sit back and allow life to carry on in those whom God has chosen. The Bible however allows for no such thought. For whilst we are not in a position to influence our calling, or the degree of gift we have been given, the Bible makes it quite clear that we do have responsibilities with regard to our preparation for service. One interesting reference goes back to the pottery metaphor we saw earlier, this time from 2 Ti 2:21. The suggestion here is that we can be a vessel used for honourable purpose if we are purged from those things (in this context profane babblings and youthful lusts), which would otherwise hinder our service. It is particularly illuminating that the verse doesn't detail things that should be there but things that shouldn't. It strongly suggests that if we are to be of value to God then we need to get a handle on our own sinful natures.
Further searching will reveal that there is also a positive side to God's selection of men. David was a young man, not really the sort you would want to lead a patriarchal warrior society but he had one vital qualification. This is given to us in Acts 13:22, his desire was to find the heart of God and he was prepared to be obedient.
Of course the ultimate example of fitness for service must lie in the Lord Jesus himself, and we find a very illuminating statement in Ps 40:8 I delight to do thy will, O my God: yea, thy law is within my heart. Whilst written by David, Hebrews 10 shows that this section of the Psalm is clearly Messianic. We find in the Lord someone not simply prepared to do the fathers will but someone whose delight was to do the Fathers will. We also discover why, it was a desire for service that came from the heart.
I suspect this desire from the heart is really the key that unlocks the question of how Daniel was so successful. In verse 8 of chapter 1 we discover that Daniel had purposed in his heart not to defile himself. The lack of defilement is of course a pre-requisite too (as we saw from 2 Timothy) but I'm sure the real action came from this purposing in his heart. At the very core of his being he knew whom he wanted to serve, this heart action from Daniel provoked a heart action from God.
I believe it is this relationship with God that underpins the career of Daniel rather than the aptitude he naturally had. I also believe that we can apply this to ourselves. Irrespective of our natural aptitudes or past histories, if we purpose in our hearts to avoid defilement and to serve Him then he will find a profitable sphere of service for us. Exactly whom he will choose, for which service, and for what reasons is a secret best left in the mind of the only wise God.
De 29:29 The secret things belong unto the LORD our God: but those things which are revealed belong unto us and to our children for ever, that we may do all the words of this law. (KJV)