WARNING: The following contains spoilers for Naruto: Shikamaru's Story -- Mourning Clouds by Takashi Yano and Masashi Kishimoto, available in English from Viz Media.
In the Naruto series, Shikamaru went from being from a lazy ninja who considered everything to be "a drag," to someone the eventual Hokage could trust with his life. It was quite a U-turn for the same ninja who quit during the Chunin Exams, leaving fans wondering if his son in the Boruto series, Shikadai, would adopt the same attitude.
As it turns out, Shikadai does lose steam early on in his career, but come Naruto: Shikamaru's Story -- Mourning Clouds, we discover what inspired him to become a shinobi again.
As his own exams showed, Shikadai had immense potential, impressing many villages who visited Konoha for the fights. But in this novel, he's disillusioned with studies and duty, even turning down an S-rank mission from his mentor, Moegi, which is a pretty big deal as it shows faith in him as a prodigy. Yet, he devolves into the same lazy mode Shikamaru had. Everything's a "drag" to him, as he says, whether it be video games, hanging out with friends or his academics. His mom, Temari, notices it but Shikamaru struggles to get through to the kid, even knowing it's a bit hypocritical as he was a slouch himself at that age. Neither parent wants their son to have a defeatist attitude.
Eventually, he gets a lecture that in life, there are hurdles one must overcome. "You have to be ready to put your life on the line for whatever it is before you can call that thing a drag," his dad tells him. "You can't use the word as an excuse to run. Once you admit that it's a serious drag, you have to do whatever it takes to overcome that thing. Otherwise, this little verbal tic of ours is just a convenient means of escape." It's great self-analysis. Clearly, Shikamaru held on to this doctrine in order to change his life. Unfortunately, Shikadai doesn't take it seriously. His parents are having marital problems and Shikamaru's always stressed; drinking, smoking and working too much, so he doesn't think the advice is worth its weight in gold.
But that all changes when he and his teammates watch the Continental Summit. There, they witness Naruto lecturing the feudal lords so a vote can be cast to stop the Land of Earth from invading the Land of Flowers. Naruto makes it clear shinobi fought wars to create an era of peace and this must be protected. He tells the old heads that moving away from the past can be a "drag" but they need to work for society to evolve. Naruto then quotes Shikamaru's same speech, encouraging the nations to unite.
Shikadai finally understands what his dad meant. Chocho even adds that her dad, Chōji, who fought with Shikamaru on Team Asuma as teens, made it clear Shikamaru was the brains of the diplomacy meeting, and once he accompanied Naruto, peace would happen. As Shikadai hears Inojin calling Shikamaru "awesome," he holds back tears.
When Shikamaru returns and peace spreads across the lands once more, Shikadai is already on an S-rank job with Moegi to escort the feudal lord from the Land of Flowers home. As they speak, while it's brief -- they don't want to seem too vulnerable -- Shikamaru is truly proud his kid's focused again. This explains why, later on in the Boruto anime, Shikadai is such a key ally of Boruto's, just as his dad is to the Hokage.