Dammnation

[I'm A Banana]

1. Because I deserve it: This works both ways. The selfish narcissist thinks that nothing is more important than what they've convinced themselves they deserve; certainly your pain comes second to their own. On the other hand, those who have been abused survive these relationships by rationalizing that abusive relationships are what they deserve.

2. Because Shar really loves me. It makes me feel special. False, of course, but imagine how seductive that would be with a god, instead of a seductive partner. Especially when they provide you with spells.

3. Because you hope that they will change. While abusers (gods, men, or monsters) will usually promise to change during the honeymoon stage, it is rare for an abuser to change while still in a relationship.

4. While you are not the cause of the abuse, it's less scary if you're to blame, because then, at least, you're in control. So you hope that by acting better or more in line with their demands/commandments, you'll make everything good again.

5. It's not so bad and everybody does it. False, of course, but possibly true to that person's life experience.

6. They draw a link between love and violence. Again, violence begets violence. Its damage gets passed down by generation. People who were abusers were very often abused themselves. This is sad, and worth a paladin's pity, but also damning, as nobody knows better the damage such behavior can do and not everyone who was abused becomes an abuser.

7. Hopelessness: The idea of being happy without your current deity may seem impossible now.

8. Gender or racial ideology. You're falsely made to believe that it's your biological fate to be treated this way as a drow male.

9. Embarrassment and shame often keep people in an abusive relationship, with gods or humans, for quite some time.

10. Financial dependence. All of your professional contacts are with this god. Where else are you going to get spells? Plus, you've made all these enemies. You need those spells. If anyone good finds out about your relationship with this god, they'll drop you socially, attack you, arrest you. Where else are you going to go?

11. You feel you have a lack of supportive relationships. The good guys will never believe you, you fear they'll smite you, and everyone you care about you've alienated or left to be with this all-consuming god.

12. Fear: This all-knowing, extremely powerful god will be ANGRY if I leave.

13. You're in the hands of an angry god, but at least you're not alone.

14. You're loyal. It's your best trait. It's what makes you feel good about everything you've done and you've stayed that way despite everything that has been done to you.

15. GUILT. With gods, we're dealing with world-class manipulators pulling the guilt-trips of a lifetime. Evil deities are world-class sociopaths.

16. Can you say, self-medication? Dependency on drugs or alcohol can play a part in staying in abusive relationships.

I can't give you any XP right now but AWESOME ideas for some nuance role-playing there. I'm nicking probably all of these for my next slew of villains. HIGH FIVE, DUDE!

 

[Particle_Man]

Another motivation: Vengeance!

Joe hurt Dan and Dan really wants to get Joe back, NO MATTER WHAT! Sign a deal with these dark powers for guaranteed revenge on Joe? Fine by Dan.

 

[Cyberhawk]

Here are a few more possible reasons:

1. The evil gods lie like a rug. They whisper all sorts of rewards and say that all the rumors about going to the hot hurty place after you die is just propaganda spread by the jealous goody two-shoes gods.

2. Necessity. Much like the sailor example before, if the god directly controls (or you think he/she controls) something that is fundamental to you then you start worshipping out of fear or a sense of practicallity. Live in a fishing village? Pray to Umberlee. Live in a land riddled by earthquakes? Talos will absolutely have a shrine and priest. Heck, you may even find some of the very rare good-aligned clerics to evil gods. (or at least the Lawful Neutral ones)

3. Revenge. The enemy of my enemy is my friend. For some reason you think Pelor is scum..well it just so happens there are some very powerful deities who oppose him. And if you shoot some prayers their way they will give you magic and you actually will be hurting Pelor (by giving spiritual energy to his hated enemy).
In these cases it may well be more of "we're NOT praying to Pelor!" instead of "we love praying to Vecna!"

Just my 2 coppers...

 

[Amphimir Míriel]

Essentially, you're asking why people get into and stay in abusive relationships. There's a few notable differences about an intimate faith-based relationship as opposed to a physically intimate relationship, but, consider the parallels.
{snip!}

Wow... Now I want to run an adventure where they have to rescue a cultist and deal with all these feelings