This was an interesting thread. I always find these labels and rule play interesting. I'll never understand the humiliation or self-loathing angle of this lifestyle. Not judging anyone, just find it a personal turn-off. So, I've been both a bull and a cuckold. Here's my perspective.
First, as a bull. I have a general desire to know everything and find the sexual behaviors of people to be a very intimate and limited knowledge to be explored. What I enjoy about being a bull, more than just being with a partner, is being granted that access and trust to knowledge and experience which is reserved to a severely limited group of people. I enjoy affairs for much the same reason. The other reasons for enjoying being a bull sort of overlap with being a cuckold.
As a man who enjoys certain homosexual pleasures...no one is the best at everything and there are different kinds of pleasures to explore. There are three major things I get out of watching my wife (or partner) with another man:
1. First and foremost is pure hedonistic indulgence. After being married for a while compliments can start feeling more like your mom saying you're the handsomest guy in school than a real critique and appreciation of ones beauty as compared to the general public. Not that the compliments and desire are false, but more like they're biased. Receiving compliments in the real world is satisfying to a degree, but going through with it and having that sexual encounter is very validating. I like that validation and want my partners to be able to receive that validation too.
2. No matter how "good" someone is, there is always a different "good" out there. Taking a truly monster cock is challenging and, therefore, a different kind of rewarding. In fact, the majority of monster cocks I've taken didn't have much skill beyond "feel how impossibly huge I am!" Watching my partner accomplish that is what I like about truly hung bulls. When it comes to other bulls my wife picked out, I was interested in learning more about her preferences and the various things which make her feel good.
3. Finally, reclaiming my love. There is nothing more satisfying than watching a man (or woman even) leave your partner utterly drained with pleasure and having that partner turn to you for that final kiss, or for a little more, or for that fall asleep cuddle. It really makes you feel they're with you for ALL of you and not just because you thrust particularly well, were born with certain traits, or work out regularly. It reminds me that we're dynamic people who have chosen to be with each other and the totality of someone's being outweighs any individual flaws or "less thans."
But, to answer the question of what makes me horniest:
As a cuckold: When my wife asks me to be in her after the bull has finished. There's nothing I love more than that gentle, passionate, lovemaking to cap off her wild passion and to reclaim my love.
As a bull: When the wife "breaks the rules" for me. Which is honestly probably against the whole "culture" or most of this lifestyle...but it's personally validating. Although, as I've read more about the humiliation and self-deprecating parts of the lifestyle...that's probably all part of the role-play.