The Jedi learn endurance early in their lives. They learn how to accept and push past their bodies' limitations, how to reach deep inside themselves for strength they did not think they had, and to take strength from the Force when even those reserves were at last depleted. But even a Jedi cannot go on indefinitely, and Obi-Wan Kenobi had reached the end of his strength.
With several ribs newly-healed and still bearing many of the bruises and marks he had sustained at the hands of Count Dooku he had been sent to find General Grievous and bring an end to the war. He had killed the droid commander, once again not emerging unscathed, but there had been a larger battle to be finished, and he had pushed on, hoping for rest at the end of it.
Instead, he had been shot at, not by the enemy, but by his own allies, and by a man whom he had once thought a friend. He had used the Force to shove the missile to the side, saving himself from death, but the impact of the shot and the debris that showered them had been too much even for his sure-footed steed, and they had tumbled from the cliff down into the water below; a fall that would have ended in death for Obi-Wan Kenobi if not for a judicious application of the Force to slow his decent.Thus he had been physically weary even before the emotional blows had started to rain down upon him. The loss of the other Jedi masters to the clone troops' inexplicable betrayal was the first, the death of the Jedi younglings the second, one so powerful that it threatened to bring Obi-Wan to his knees in a way that General Grievous's punches never could. Yet even that was nothing compared to the pain of the realization that Anakin, the boy he had loved and the man that had been his closest friend and companion, was responsible for the destruction of everything he had loved. It was almost too much.
But he was a Jedi, and the only thing he had left in the galaxy was his duty. He did his duty; though it nearly tore him apart, he did what he had to do. Later he would ask himself why he hadn't made that killing blow, why he had turned and left before the man who had once been Anakin was truly dead, but at the time he only felt relieved that the task was done, that he had brought to Anakin the one measure of peace which was still available to that tortured soul, and, selfishly, that he didn't have watch death take that once beloved face. He had wanted to cry, then, to collapse and loose himself in tears for the fate of his brother, but Padmé had needed him, and he had once again turned himself to the task still needing to be done. He was strong, for Padmé, and then for Luke, the tiny newborn who needed a family, needed somewhere safe where he could grow into manhood and the awful responsibility which now lay on his and his sister's shoulders. Obi-Wan, too, felt the weight of that responsibility, but as Beru took the precious bundle from his arms it was the weight of loss that he felt most strongly. At that moment he wanted nothing more than to snatch Luke back, to keep him for his own, to hold close the only person in the world who was left to him.
Instead, he watched as Beru moved to her husband's side, as Owen leant down to peer into the baby's face, as the Lars family, now one member stronger, stood on the gentle ridge and gazed at the brilliant Tatooine sunset, an image so poignant to Obi-Wan that for a moment he could scarcely breathe. And then, stiffly and silently, he had wept. But he didn't cry for Anakin now, nor did he cry for the heavy fate which awaited Luke. He didn't cry for the innocents who had died, nor for those he knew would die in the face of the new order of the galaxy. He cried now, selfishly, for himself, for his own losses that in the grand scheme of the galaxy were so small, yet at the moment seemed so very overwhelming that no amount of Jedi self-control could withstand them.