Beyond Reason
Shikamaru was a born genius. Throughout his childhood, he easily trumped his peers by a wide margin, and was able to crack ciphers which most adult ninjas struggled to understand. But the achievement he prided himself most on was his father's admission about him being his toughest living opponent in shogi. In shogi, just like in real life, a great player can have half as many pieces on the board as their opponent and still be able to snatch victory from the jaws of defeat. Maybe that is why Shikamaru loved shogi so much.
Unfortunately, Shikamaru was born too damn brilliant for his own good. While his peers were playing 'ninja', he was estimating his chances of getting a promotion to jonin, as well as avoiding becoming permanently crippled along the way. While his academy classmates were eagerly competing for the title of top student in the class, he was indifferently maintaining his carefully calculated place in the overall class rankings, which was barely enough to finish the academy. Nevertheless, Shikamaru was still a child, so once in awhile he allowed himself to relax and watch the clouds. He dreamt about becoming Hokage and changing the world, or at least Konohagakure, for the better. But Shikamaru's resolve to start training properly always crumbled after he heard yet another rumor about a vicious Naruto-beating, and how that 'old bastard Hokage' was preventing faithful villagers from finishing the damned demon fox off. If the Hokage couldn't even improve the shitty life of one of his own favorites, then the chances that the Hokage could change something for the better were slim to none. Just like shogi, life provides a few opportunities, and makes many more impossible. Maybe that is why Shikamaru hated shogi so much.
Until the fateful day of the Uchiha massacre, Shikamaru had been living a simple life - he had made the least possible amount of effort to fulfill his duties, and patiently waited for fate to make a move for him. On that day, it seemed that fate decided to accept his challenge.
Shikamaru was sent by his mother on a routine task to collect ordered ninja equipment from the Uchiha district. After he collected the equipment and started to head home, barriers appeared around the compound, catching him by surprise... and then the massacre started. In that moment Shikamaru realised one simple thing - today, he was going to die. The screams of dying people, clawing at the impenetrable barrier, suggested that there would be no survivors. Shikamaru tried to run among the frenzied crowd of civilians, but barrages of kunai killed most of them instantly. The sight of people falling and dying, even from the slightest wounds, forced Shikamaru to make the only decision which could postpone his own death; using his own kunai, Shikamaru stabbed himself in the gut.
Lying in a puddle of his own blood, Shikamaru watched how the masked assailants methodically finished off the rest of the fleeing civilians, not paying any attention to just another bleeding child on the ground. As the edges of his vision blurred, Shikamaru absently wondered what kind of poison they were using to kill everyone with such efficiency. Although his subconscious was still frantically, instinctively, looking for any opportunity to survive, Shikamaru already knew the inevitable - he was finished. Either he would move and attract the attention of the assailants, or he could remain motionless and soon die from the blood loss. Even if he got lucky, and survived until the barriers fell, he would not be able to get to a hospital quick enough. So, despite the fatal, desperate situation, Shikamaru was doing the same thing as usual - biding his time and dreaming about the impossible.
Still to this day, Shikamaru didn't know what made him get up, through the pain and weakness, and slowly crawl away from the Uchiha district when the barriers fell. Maybe it was the blood loss that had left his mind too dazed to make logical decisions. Maybe, in that moment, fanciful things like a desperate ray of hope actually revealed themselves as stronger to him than cold logic. In any case, a half-dead Shikamaru broke into the first dwelling outside of the massacre zone, shouting for help. As he recognized the tenant as a retired medical-nin, he felt an almost completely alien sensation in his logical mind - hope. Hope that maybe, just maybe, there is no hidden meaning behind playing shogi, it's merely a board game for developmental strategic thinking.