I also got one not long ago, a round dial about 3 inches across, on my quads a few inches above my knee that was exceedingly painful, an area where the pain charts seem to suggest is not usually painful.
The most painful for me is when when the outline is done and they go back to do the shading/texturing and the skin is already angry. That's a good time to stop if the tattoo is sufficiently large.
Hence, you have a pain map of sorts, based on things such as the dermatomes. Utility of the skin also comes to mind, the hands, although 'tough' have a lot of nerves and, allegedly, a job stopper hand tattoo will be extremely painful. Same with the chest, which is 'underbelly' and not 'defended', whereas your shoulder will not have the same 'evolutionary' choices.
Coupled with that is the evolving technology, in the olden days, tattoo machines looked like they were held together with rubber bands, and people swore by these contraptions. Nowadays you have something more like a glorified electric toothbrush, with most artists just preferring the modern tools, that also hurt less. Aftercare has improved too.
Different artists will also impose a different level of pain. Pain also is 'in the eye of the beholder', for your first rodeo you probably had a certain amount of adrenaline going round, for the 'other shoulder' you won't have this 'natural pain killer' to quite the same abundance, so it might hurt more than you expect, rather than less.
Nonetheless, dermatomes, you need to go down that rabbit hole! I never knew of the things until recently.
1. The location of tattoo, some regions of skin are VERY sensitive.
2. The skill of the tattooer. How deep do they go, how fast do they pull, etc...
3. The size of the lines and shading A fine line tattoo feels very different that a block shaded piece. (One needle vs up to 7)
4. The style of the tattoo. Black and gray realism tattoos require the artist to go over an area many times. Going over an already inflamed piece of skin feels way worse than fresh skin.
The worst I've experienced is on the elbow; it felt like my entire skeleton and nervous system were being jackhammered.
I almost missed that there were 4 colors until the second chart