Good question. I’ll bet if it was a fig leaf you would ask, “Why not an olive leaf?” — and that would also be a good question!
Rashi (Rabbi Shlomo Yitzchaki, 11th century France) explains that by bringing back a bitter olive leaf in its mouth, it was as if the dove was saying to Noah, “Better that my food be bitter and from Hand of God, than sweet as honey and from the hand of man”.
During its stay in the ark, the dove had been obliged to rely on Noah for food in order to survive. It brought back a bitter olive leaf — which it would not normally eat — to express an idea. The most bitter food eaten in freedom is sweeter that the sweetest food eaten in captivity.
In this spirit we welcome the freedom and homecoming of Gilad Shalit and may his “food” only be “sweet”, as we pray for happiness, success and peace in the Middle East and the entire world.