pain and power (a perfect combination)
For Amycus, it's about the pain.
He loves knowing his victims are hurting, and it's all his fault. All he has to do is cast a simple Crucio and they're screaming in agony, the pain of a thousand knives coursing through their bodies. They always beg him to stop, to show just a little mercy, but Amycus never does. He will keep at it until they're numb and barely even feel the pain anymore.
At the time, Amycus will laugh at his victims' predicament, being so wrapped up in the euphoria of the moment that all he can do is laugh and proceed to extend the curse. But later, it will hardly matter. The only thing that will stick with him is the best part, the pain.
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For Alecto, it's about the power.
She loves knowing that it's all up to her whether they will ultimately walk free, or if she'll get a bit carried away and keep at it until her victims lie in a lifeless heap at her feet - where they belong. All she has to do is cast a simple Crucio and they're at her mercy, hoping their torture will end before it's too late. It would be so easy to lift the curse and leave it at that, but it wouldn't be any fun. They wouldn't learn their lesson - they aren't in charge of their own fate. That has fallen on Alecto.
At the time, Alecto will laugh at her victims' predicament, watching them try to regain control and knowing she will never relinquish it. But later, it will hardly matter. The only thing that will stick with her is the best part, the power.