I'm assuming you're inquiring about pre-Meiji period, because the post-war suicides of Japanese military personnels was a whole different story.

What we need to understand is that the practice of seppuku is almost never a ritual practice. Stories are well, just that. The problem is that the famous stories we hear in the Western world might have been a third, or even fourth hand stories. During the Edo period, people wrote chronicles of their clans or famous clans that had died out. These stories are often created with very strong political purposes. Kuroda Kanbei the strategist? Well, the Kuroda wanted to make their face a little prettier by pretending they weren't just one of the many kokujin commanders under Hideyoshi. Ieyasu was helped by Iga ninjas to travel back to Mikawa? Well, samurais in Iga were out of jobs and wanted the neighbouring hans to hire them, so they said "hey, we made the Edo bakufu possible! Shouldn't we be compensated?". These stories then went through contemporary Japanese pop-culture interpretations and creative liberties. then finally the Western media takes its own liberty with interpretations.

We must look at the contexts in which suicides are committed to determine their nature. Different cases had different contexts, but it almost always boils down to one thing: they're gonna die either way. I think here are 3 main examples of when seppuku takes place:

1. If your castle's besieged. Who knows whether the enemy will take pity upon your life. If the enemy wants to take your land as his own and consolidate control, then a previous owner poses as a threat (and probably will be killed). Famous examples like Bessho Nagaharu, Azai Nagamasa, Matsunaga Hisahide.
2. If you're ordered to commit suicide by your lord. This is because usually your lord will not end your whole family, and will at least give some land to your offsprings. Whether out of pity, consideration of your family's past efforts, or simply preferring to use the clan's influences within their land. Even if that's not the case, your refusal to suicide might just mean they'll drag you out and execute you. That's not much better either.
3. Junshi, the practice of committing suicide when your lord dies. This is a slightly more complicated case that involves tradition, consideration for the next generations, and peer pressure. However, the main goal, in the simplest form, is to ensure the best treatment for your next generations. When the new lord comes onboard, their families would go "hey, my father killed himself for the previous lord! Shouldn't his loyalty be rewarded?". It's in a way, a very violent, forced promotion for your family. Because those who committed suicide with their lord's passing are seen as more loyal and are thus rewarded, those who didn't do it easily become the target of punishment. So, the practice of Junshi has a peer pressure element to it (in that if someone does it, there's gonna be other people who also do it).
4. (BONUS) Suicide as a way to change your lord's terrible behaviours. There are numerous examples of people doing it, and it shows just how loyal they were to their lords. Hirate Masahide is perhaps the best example. However, this is not supported by any valid historical document, and neither were almost all cases.

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TL;DR: Most seppukus weren't ritual in nature, and were mostly depended on their very real circumstances - be impending death either way or consideration for your future generations. If a commoner didn't have the same circumstances, then of course they wouldn't commit suicide.

edits: typos