My horse was very girthy when I got him. He'd kick, nip, dance around, etc. I'm not 100% certain of the cause or what eventually made it go away, but I'll tell you my best guesses
I got my horse from his breeder- he had been returned by his previous owner when she couldn't afford to keep him. She said he had never been girthy when she had him, and someone else who knew the breeder said he did cinch up very quickly. So one possibility is that the breeder caused the girthiness by habitually cinching up too fast.
He also came with poorly maintained feet and had heel pain (diagnosed with a response to hoof testers); my saddle fitter/acupuncturist (who is also a vet) said one of the "ting" points for the heels is on the girth line. She put one needle in each of his heels as an experiment and while they were in he was MUCH less sensitive on the girth line. She said acupuncture wouldn't provide lasting relief until the heel pain went away. I have a very good farrier for him, and he now no longer responds to hoof testers.
I always girth up very slowly, tightening one hole at a time, alternating between left and right sides. I also frequently (but not always) give him a treat each time I tighten, and correct him anytime he threatens to kick or nip. I've also had his saddle fitted to him and have an anatomic girth.
A few months ago (when I'd had him just under a year) I noticed he wasn't girthy anymore. I can't say exactly when it stopped, but I'm very glad it did. He no longer acts uncomfortable when I'm tightening the girth, and he doesn't even pin his ears when he sees me bring over the saddle.