Need

What is need?  We all have needs, but what makes a need worth serving and worth dedicating a year of my life to? 

This morning I was at Target with my mom and while we were checking out, the cashier asked me about what I was doing this fall (assuming it was back to school shopping, I think) and when my mom told him I was going to be doing missionary work in Los Angeles, he looked completely shocked, stopped what he was doing, and said "LA? really??"  I said yes, and he said something about being surprised because he always thought missionaries served in places like Indonesia, Africa and South America, not in the US.  He then added, "I guess people here need help too."  But it seemed like a consolation statement to me.

Why do we so easily assume that the needs of those outside our country are so much greater than those within the US?  I am guilty of it too, so I'm not trying to accuse this guy of anything.  But, what does it mean when JVC states that its volunteers "serve where the need is the greatest"?
I'm not really sure what word or idea I am wondering the meaning of,but this brief conversation really stuck with me and has left me confused.  More on this later with more insight hopefully!