Let's see if I can put a few in a list...
Hope this gives you some ideas!
- People wanting power to overcome a foe (good or evil)
- People wanting power after losing something/someone important (good or evil)
- Someone being tricked into it (see Thaumaturge questline)
- Inquisitive nature (usually a misguided good scholar/summoner)
- People wanting to make a trade/gain knowledge (good or evil)
- People trying to bring someone back to life (good or evil)
Hope this gives you some ideas!