Chazal metaphorically describe the giving of the Torah at Har Sinai as a "wedding ceremony" - between Hashem and the Bnei Yisroel. Rashi there relates how Hashem's shechinah - divine presence, went forth to meet Bnei Yisroel, just as a Chosson goes out to greet his bride. Just before the Chuppah, Hashem tells them "והייתם לי סגולה מכל העמים" - "and you shall be my treasure among the nations."

     R' Henoch Leibowitz zt"l elucidates this pasuk. When a person receives a great deal of love from Hashem, his attention and thoughts become involved with this relationship, to the exclusion of anyone else. While he becomes aware of the love bestowed upon him, he loses some appreciation of being selected from a large field of candidates. He forgets how special he is in Hashem's eyes and Hashem's love is no longer as meaningful to him, because to a slight degree, he feels there is no one else for Hashem to love. 

     It is this phenomenon that we see from the words, "my treasure from among the nations." Of course, we know that there are many nations on the earth, but this knowledge becomes clouded in our emotional enrapture. We lose sight of the fact that Hashem could shower his affection on so many others instead of us.