Why we're selected by God to do what we do

I am currently working my way through CS Lewis' sci-fi trilogy (Out of the Silent Planet, Perelandra and That Hideous Strength). Though I'm not a big sci-fi fan (admittedly, I haven't been exposed to much), I am of Lewis. The series is filled with his usual brilliant wit and colorful turn of phrase. I came across the following quote this week and have been thinking about it ever since. In this particular scene, the main character is explaining the reason for which he was selected for the task at hand. Surely his reply explains why any of us are selected for anything. I am certain Lewis is right here and, one day, we will laugh at any seriousness with which we treated ourselves.   

Don’t imagine I’ve been selected to go on to Perelandra because I’m anyone in particular. One never can see, or not till long afterwards, why any one was selected for any job. And when one does, it is usually some reason that leaves no room for vanity. Certainly, it is never for what the man himself would have regarded as his chief qualifications.
— CS Lewis, Perelandra, 22