WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 173, "The Secret Behind the Underground Room," now streaming on Crunchyroll.
In the Naruto series, Shikamaru Nara was always the title character's closest ally. They grew up preparing for the ninja exams, and while Naruto was in a team with Sasuke and Sakura, it's Shikamaru who most enjoyed his annoying ways, showing him patience unlike any other and helping Naruto grow past feeling alienated.
It's why, as he broke out of being an outcast, he kept Shikamaru close as the latter was the best military strategist in Konoha. Now, taking on the Hokage's role, Naruto has Shikamaru as his main adviser, bringing their relationship full circle after so many years. Based on this history, and given what we're seeing in Boruto, Shikamaru's son needs to join Naruto's son ASAP as there's a similar friendship, both on and off duty, that's developing.
Few people have tolerated Boruto as he's a tad rash and petulant, maybe even more so than his dad. Sarada and Mitsuki work around his ways, accepting him for who he is, but it's really Shikadai that gets him. It could be that this was passed on from father to son, as he watched his dad work so well with Naruto, but Shikadai can tell there's something special about the kid, too.
Given their bond, it's no surprise Shikadai understood Boruto's problems in training and helped him evolve his Rasengan at a time no one else knew how to. This just proves how tight-knit they are, with both tempering each other as Boruto inspires Shikadai not to be lazy as Shikamaru was as a teen. Now's the perfect time for Shikadai to join Team 7 on a temporary basis, too, as they need to hunt Deepa.
Shikadai faced the villain after he crushed Boruto's team and placed Mitsuki in a coma, so his tactics would be most welcome. He knows how to survive fights as he's more defensive-minded with his shadow techniques, which fits Boruto and Sarada's offense. This would add a nice dimension to the team as they really haven't had that lead strategist, and this ups the maturity level of the crew as Shikadai understands everyone's abilities and how to sync them all up in the field. It harks back to his dad, as well as his grandfather Shikaku, who sacrificed himself in the war against Kaguya.
Fans always wanted more partnerships with Shikamaru and Naruto, so this is a chance for the next generation to get it right. Shikadai can also remain with his team and just work on loan from time to time according to the mission, which Konoha does allow.
The point is, he fits better with Team 7 and feels like the right addition, especially with Deepa and Kara running sinister schemes to take out all the great nations, while Mitsuki still has his own family problems. This would also set Team 7 up for the future if Naruto or Sarada wanted to take the Hokage's spot, with Shikadai ready to help out as main counsel.