Brothers
You were my brother Anakin. I loved you.
They had been as close as brothers. Closer. And that, really, had been part of the problem. Obi-Wan knew he had been too young, too inexperienced to be responsible for a young child, and especially one who was suffering from the loss of his mother and the fear of being in a new place, away from the only home he had ever known. Especially when Obi-Wan himself was suffering from the loss of the only father he had ever known, but he had given Qui-Gon his word, and so, no matter how uneasy it made him feel, how out of his depth and unqualified he felt himself to be, Obi-Wan knew he could never break his word to Qui-Gon.
They were too alike in so many ways; both had been taken on as padawans when no other Master would take them. Both were excellent swordsmen, some said the best the Order had ever seen. Both were strong willed, stubborn, even. Both listened to their emotions more than a Jedi should, though Obi-Wan was better at suppressing them.
They were too close in age for Obi-Wan to ever truly be in charge, and especially since he had only just become a Jedi Knight when Anakin became his padawan. It was almost inevitable, really. They were too much like brothers, forever arguing and getting each other's nerves, forever laughing and teasing each other. Obi-Wan tried to act responsibly, but with Anakin around he always failed. Anakin had a way of always managing to persuading him to break the rules, or ignoring his instructions entirely.
They loved each other, they spent all their time together, even after they were no longer the Master and the Apprentice. They became legends throughout the galaxy together: The Negotiator and The Hero With No Fear. Obi-Wan was one of the few who knew that his beloved brother was full of fear, one of the few that saw past his tendency to act like he had no fear and do insane things like jumping out of Coruscant speeders. Anakin was one of the few who Obi-Wan couldn't negotiate peace with, whether it was in their playful banter or the final, painful, confrontation.
They were brothers, and there was only one conclusion that Obi-Wan could draw from that: where he had failed with Anakin, he would not fail the only responsibility he had left, the only thing he had left to live for.
He would not fail Luke.