For the fistee, part of it is the physical sensation, which can be tremendous, particularly when the prostate is stimulated. For the fister, there is the tremendous kick of being inside someone so deeply, where you can feel the warmth and the strength of the pulse. For both parties, there is the mental thrill, the intimacy and the trust. Trust is vital to fisting. If you want to be fisted you should be able to say, hand on heart, that you trust the fister to look after you.
Some people can make that decision a few minutes after meeting.
Others could take months. How long it takes to decide does not matter. The trust matters. Without that trust a fistee will find it difficult to relax. If he can't relax he either won't be able to take the fist or it will hurt badly.