Having an agent or an editor email you back and say, "It's just not for me," feels just as bad as if the email had read, "This is garbage. You suck. Quit." To me, that is really what it feels like, even when I know that those people (most likely) don't mean it that way.
Regardless of how much or little success one has as an author, rejection still hurts. Someone just told you that something you put your heart and soul into isn't right for them. Even if you understand that your writing isn't for everyone, it still hurts, especially if you thought it was right for the to whom you sent it.
Case in point: yesterday, I received a reply to a query for THE FIFTH. The agent said, "I appreciate your considering me, but I just didn't like where the story was going."
It makes me wonder, where did she wish the story had gone? I had some other ideas, but the one I went with felt like the right path.
The best thing I can take away from that type of rejection is that at least it was somewhat personalized. I consider getting personalized rejection a step in the right direction because at least it means the agent thought about it for more than 5 seconds before saying no.