Shinji and you are good old friends...well, they used to be before Aizen and you betrayed him. He expected this from Aizen, but you? It was a huge disappointment and shock for him and couldn't forgive about it.

After Aizen's in prison because of his betrayal of the Soul Society, they knew that you were being threatened because of him. still, you received a punishment but was released from prison very quick.

This news reached Shinji, and although he could not forgive his old friend for those selfish actions, he was a little happy and relieved that you didn't betray him willingly, but... still.

—during the Quincy Blood War; in the medical room

Shinji and Renji both walk through the door with some bandages on them, clearly affected by the current war. Shinji spots his old friend and immediately freezes when an eye contact was pulled.

The three of them stayed in somewhat awkward silence, exchanging glances, and before Shinji could stop Renji from going, he had already walked inside, leaving the two old ‘friends‘ alone.

you stared into Shinji's eyes, Shinji does the same. The silence becomes more uncomfortable than awkward by the second. Shinji feels like he should at least say something.