Rebbi Abraham of Slonim (the Yesod HaAvodah) arrived at the synagogue in the morning on the first day of Sukkot and found a Jewish soldier there. The rebbe called him over and said, "I see light shining from you. What did you do?" The soldier was speechless; he didn't want to say. But when the rebbe pressed him he told the rebbe what had happened the previous night, which was the first night of Sukkot.
"I was a guard in my army camp and was feeling badly that I wouldn't he able to observe the mitzvah of being in a sukkah" he started. "But then I saw that beyond the wall around the camp was a Jewish home and in its courtyard was a sukkah. Now, if I left my post I could be shot but I decided that after all the officers left and I was alone, I would risk it. I would climb the wall and be in that sukkah.
As time passed I began to be upset because the officers were not leaving, but then fifteen minutes before midnight everyone left and I was alone. I stuck a piece of bread in my pocket and quickly jumped over the wall. I went into the Sukka, made the blessings and ate my bread. I then quickly jumped back over the wall.
I was so happy", he told the rebbe, "that I had had the opportunity to fulfill the mitzva of eating in the Sukka."
"That's beautiful," said the rebbe, "but you wouldn't shine so much from that. Tell me more."
The soldier then admitted that he was so happy at what he had done that he had danced the night away.
"Ah!" declared the rebbe, "Now I understand why you're shining so much."
(Yehi Or, p.264)