Hmmmm I'm not sure if I fully understand it myself. But I'll try.
The way I see it, NTR fetish is about the same as cuckolding. There's a certain thrill about seeing another guy bring the object of your affection to orgasm. I have no idea why but for the person stuck observing, it brings out a very strong state of arousal. There are a lot of stories about guys who can't get aroused normally, unless it's this type of cuckolding scenario. So I'd guess that for those types, it really is the most arousing experience for them. I've also heard it might come out of insecurity. Feelings that you can't properly pleasure your woman, yet another guy can make her lose her mind (or woman, however this theme is quite rare). Despite that feeling of inadequacy, it's an opportunity to see the person you love experience pleasure you didn't even know about. I think, maybe if you care enough about a person, you want them to feel good even if it's not you in the action. Obviously a normal person probably wants to be the one doing it, but I think someone who's really deep into cuckolding just comes to accept that someone else is the right person for the job.
For a long time I've been extremely disgusted by NTR, but I've come to accept that some people genuinely really love the genre. I can't speak for that type of person, but I've come around a little bit. I still experience the same feelings of jealousy, discomfort, and heartbreak as I used to. But I do think that the raw lust and pleasure depicted on the page can be quite exciting. I really have to try not to get too emotionally invested in the stories to see that it's good hentai. Because that's what it is. Hentai is supposed to be erotic and exciting. Some hentai does happen to be happy and heartwarming, but there are definitely other ways to make it sexy and thrilling. NTR is one of those. I think maybe that's a good way to look at it. NTR is kind of like a "taboo" that explores a thrill that you just can't get out of a normal romantic love scenario. It's its own brand of sexy and for some people it's the best.