RPG is a complex game. It has multiple elements which could make it addicting.
The first and easiest one is numbers go up from levelling up. Defeating enemies yields XP which allows levelling up. With level up, player gets stronger and could defeat stronger enemies which yields more XP. Repeat the cycle.
The second one is story. More often than not, RPGs have story which starts simple but has many mysteries. As player progresses, mysteries unfold one by one. The reward from unfolding story is addictive or at least compels player to play longer. However, unfolding mysteries could also reveal more mysteries which extends the playing time.
The third one is skill and or spell unlock. As player gets higher level, more skills and or spells are unlocked compelling player to play longer to unlock more.
There could be more but those 3 are the ones I have in mind ATM.

























































