I'll bet you have a fairly narrow view of suffering. I'll bet when you read that caption you thought about that beautiful girl spanking you or taking a belt to your ass. There are other ways a submissive man can suffer for a domme. In fact, I'd argue that most dommes find it far more interesting and arousing when a man is willing to suffer in non-physical ways.
Here are a few examples:
The Agony of Orgasm Denial - Men want to cum. Men crave orgasms, in fact. They're designed to crave orgasms, to spill their seed, to spread it around. To be denied that most basic of pleasures...well, that is a form of suffering. It's variable, too, as your suffering increases the longer you go without an orgasm. Would you be willing to a month without cumming while serving her as a submissive? Would you be willing to go two months? Would you put your pleasure aside for the thrill of serving her?
Financial Suffering - This is, of course, a controversial topic, but I offer no commentary on it other than to say that some women find great pleasure in a man's willingness to suffer financially for her benefit. It's not just being generous, either. No, it's about giving up something to benefit her. It's about directing some of your monthly budget - say what you spend on entertainment - right to her. It's about your willingness to go without something that you enjoy so that she can have something she enjoys. That's where a true findom finds her pleasure. Would you be willing to suffer in that manner?
Doing Gross Stuff - Eating cum. Licking the bottom of her boots or her dirty feet. Putting your tongue deep in her asshole. Licking her pussy after she's peed. None of that is particularly pleasant. All of it would be called gross by some men. Very few men would find genuine pleasure in any of it, and thus the suffering. Would you be willing to do those things for her?
Surrendering Your Sexuality - I'll bet you don't want to suck cock. Not on a biological level. I'll bet you don't go on Grindr and look for dudes to blow. I'll bet you don't want to get fucked up the ass by a man or take a massive strapon cock anally. I'll bet you would find it unpleasant to do any of those things. You'd suffer, in other words. Would you be willing to do any or all of those things for her? Would you suffer in that way for her?
Public Exposure - What if you went out with her and her friends and she told all of them that you had a small dick? What if she told them that you want nothing more than to watch someone else fuck her so you can eat his cum out of her? What if she told her friends that she doesn't let you fuck her, that she only gives you handjobs, that she feeds you your load each and every time she makes you cum? What if it was your friends she wanted to reveal those secrets to? Would you be willing to suffer that kind of embarrassment for her?
You see? There are lots of ways you can suffer for a domme. At the core of each form of suffering, though, is your willingness to do whatever it takes to make her happy, which is the thing that every domme craves more than almost anything else.
The further you're willing to go to make her happy, the more pleased she'll be with your submission.