Power
Occasionally, you have the power to mend severe emotional issues and make everybody happy. And that's the kind of power that'll make poor, emotionally-scarred Batman go green with envy.
More often than not, however, you'll simply see families or couples for months or years, during which time you'll help them cope with recurring issues, or brand-new problems that spring up over time—in other words, there isn't generally a cut-and-dried solution that fixes everything. It's an ongoing process.
In that respect, you don't have much power at all. You're not allowed to yell and scream and curse like your patients can—you have to remain composed and professional at all times. You likely won't cut off ties with any of your patients unless one of them threatens you physically, but your patients can up and leave you whenever they feel that you are no longer of service to them.
Hmm—doesn't sound like a very healthy relationship. Perhaps you and your patients should see a therapist?