Several time in the manga/anime "Instinct" have been mention.
Yoruichi - Talk about human can instinctively know how to walk, bird know how to fly, fish know how to swim and Ichigo can instinctivley learn how to do bankai.
Hollow Ichigo - Talk about the difference between the King and the horse is Killer Instinct. That Ichigo doesn't have the killer instinct and he used his head to fight and he need some reason to fight. You need killer instinct to gain power.
Kenpachi in ichigo's subconscious explain that ichigo instinctively seek out battle to gain more power, or to gain power to battle more, that ichigo born to fight and doesn't need any other reason
Concluding the battle between Ichigo and HIchigo, Ichigo defeated him because he still have some of the instinct to battle.
Grimmjow - telling Ichigo that he didn't come to Las Noche to save Orihime, that his fightning instinct have brought him here. Shinigami and Hollow are instinctively fight each other and been like that for 1000 years.
Rose - talking about joining force to fight a common enemy is survival instinct held by all living thing
I'm sure I'm missing some, but those that one I remember. It not coincidence that instinct have mention couple of time, but does it have some important, or a purpose in the future.
It is simply a important recurring motif IMO. It will be mentioned again.
You may be on to something. I think it may be related to what was said in this thread.
Originally Posted by Minato-sama
I think we'll gain more insight into this when Isshin and Urahara explain Kurosaki family history, and who Ginjou and Tsukishima are. Ichigo was born special and I'm sure Ichigo wants to know why.