In Parshas Lech Lecha we read that Malkitzedek, the king of Shaleim, brought out food and wine for Avraham. That same place of Shaleim would later be renamed Yireh by Avraham Avinu in the parsha of the akeidah. Two names given to the same place by two great people -- which one would win out? The Midrash (parsha 56) writes that Hashem made a comprimise. He combined the two into one and thus we have the name Yeru-shalayim.
What difference does it really make what you call the place? It's still the same city, the same place on the map?
Rav Kook explains that the two names reflect two different approaches to religious development. One approach is the philosophical approach, coming to Hashem through contemplation. The other approach is the approach of tikun ha'midos, fighting off the yetzer ha'ra and the influence of evil and in doing so coming to purity. Malkitzedek is the "kohen l'K-l Elyon," the master of looking on high, contemplating G-d in the heavens. Avraham, on the other hand, saw the need for yirah, for bringing things down to earth, dealing with the world and all its temptations and imperfections and bringing G-dliness to it.
Yerushalayim is the meeting place of both worlds. It is the place where the Beis haMikdrash in heaven stands exactly corresponding to the Beis haMikdash on earth.
Rav Aviner writes that after the Six Day War he asked R' Tzvi Yehudah by what zechuyos we had earned having Yerushalayim in our possession. R' Tzvi Yehudah answered that it was not our zechuyos -- we surely are not deserving -- but rather it is a gift from Hashem.
I think it's a davar pashut that if someone gives you a gift, kal v'chomer if G-d gives you a gift, that you have to say thank you and show your appreciation.