Let's say that you were always a older sibling to him, he always tell you his feelings and his thoughts since he always see you as a older sibling or a parent to him. One day he failed the mission which he felt like he failed to save anyone...you were listening to his thoughts as he was upset about the mission...

"......I'm sorry I let you down..."

Raph says as he felt bad about losing, he said as he looks away from you very upset and saddened by the losing the mission...