Let’s take a look at the very first page of the Gintama manga. It is about Shinpachi and Tae, with their father on his death bed, talking about the decline of the samurai power and why they must hold on to the 'sword of their soul'.
The sword of your soul, or we could also understand it as ‘Gintama’. Gintama literally means silver soul, and what is silver? A sword is silver.
So here Sorachi wrote Shimura 'Ken' (剣) telling his kids to hold on to Gintama.
If that’s not poetic I don’t know what is.
It becomes even more poetic when the siblings cross path with Gintoki, the human representation of ‘silver soul’. Gintoki’s character is defined by his bushido, that’s why he doesn’t carry a sword. His soul is his sword. The Shimura siblings had also promised to never give up the sword of their soul no matter what.
Then we see how Shinpachi, Tae, and Gintoki try to protect what is important to them. For Tae, it's her father's dojo, (If I'll suffer either way, I'd rather suffer trying to save it.) for Shinpachi, it's her sister's happiness (I'd rather lose the dojo than see my sister cry). Both siblings are willing to lose something to protect the sword of their soul, just like when Gintoki was willing to broke his promised to Takasugi to keep his promise to Shoyou.
Gintoki of course has got nothing to lose at this point. The only thing he has left is his bushido. The sword that promised to protect what's important. So he helps protect what’s important to Shinpachi and Tae without them having to sacrifice anything, so they don’t have to go through what Gintoki went through with his Joui brothers when he executed Shoyou to save his friends.
Gintoki says, "what is important to my customer is important to me.", which pretty much explains why he started the Yorozuya. He tried to protect what’s important to him once, and he failed. Yorozuya is how he aims to redeem himself.
After Shinpachi and Kagura came along, he’s no longer just a sword that protects, his soul gets protected too in return. This is especially evident in Hamko episode and Memory Loss arc. Once again he has something to lose, and something to live for. Although it has never been mentioned, this is mainly thanks to Tae, for letting Shinpachi go with him and allowing these connections to happen. It’s thanks to Tae that the whole Edo is now becoming Yorozuya to protect what’s important to Gintoki. If Tae hadn’t let her brother go, if Gintoki hadn’t met Shinpachi and Kagura, none of that would happen.
Going back to the first lesson, at the end of it the Shimura siblings admit that they see something in Gintoki. And here is the first reason that convinced me that GinTae is meant to be. Tae tells Shinpachi to go find his sword with Gintoki, assuring him that she'll find her 'sword' in her own way. And just like Shinpachi found his sword from working besides Gintoki and learning from him, Tae is meant to find hers in him as well.
But that subject hasn't been raised for Tae like it had with Shinpachi at the start of Silver Soul (chapter 601). That is suspicious and interesting, especially since it is a subject fit for both siblings and Sorachi could have written that moment for Tae as well, but yet he didn’t. Why?
Because I believe it's a moment reserved for the end of Silver Soul. It is afterall the kind of poetic ending that has been foreshadowed since the first lesson.
