It may just be that I'm demonstrating my gross ignorance, but I can't seem to find a 'why' for stem-changing verbs in Spanish. I understand that there is some sort of perceived weakness in the vowel that is emphasized by the addition of stress to the syllable, and that the vowel is somehow strengthened by the shift. I just don't understand the governing principles of that shift. Why do some vowels shift and not others? What is it that says that the 'e' or 'o' in this verb will change, but not the 'e' or 'o' in that verb?
Every search I've done just comes back with a lot of people saying 'that's just how it is, memorize it', and that's not really an answer. Any help out there?