Growing out his hair is a tricky thing for Sasuke. It was very common for Uchihas, especially the ninjas who were notable warriors, to grow their hair long and to keep it long, but Sasuke mostly kept his short growing up from age 7 out of spite.
But as time went on and the truth of the Uchiha clan came out, and he decided that distance from the village was what was best for him come post war, growing his hair quickly changed from a choice out of hate and spite to a choice out of personal safety and mental wellbeing. The first time Sasuke's hair had grown long enough to touch his shoulders, he took one look at his reflection in a nearby creek and ... blanked out. All he can remember is the tight feeling in his chest, of being unable to breathe as he grabbed his kunai and hacked away at his hair until it was choppy and short; hands shaking all the while.
He can hardly take the mere mention of Itachi without retreating into himself, so looking into any mirror or reflective surface and seeing a sight that looks almost just like him? Unbearable.