I realized today that the writer-agent dance is a lot like being single and looking for a mate and in particular, like going on a blind date. Both agent and writer presumably want to find someone who makes their heart go pitter-pat, the writer so he can get published, the agent so she can make a living.
The query is like the first, blind date. Most of the time it's not going to work out. You realize you have nothing in common. You don't set her heart aflame. Doesn't mean you're a bad person (i.e., your book idea sucks) or that your dating skills (query letter) are inept. You just weren't meant for each other, for any number of reasons.
A partial is sort of like a second date. Maybe there was something she saw in you on that first date that intrigued her enough to go out with you a second time. But maybe she isn't bowled over on the second date, even if she wants to be. She realizes you're a smoker, or a Republican, or a vegan or you just don't get her juices going (your partial didn't work for her). So she turns you down for the third date (the full ms.).
The third date is kind of make-or-break. When you get to the front door, either she invites you in and you sleep together (representation) or she says something like, "I'll call you some day," ("I just didn't fall in love with your novel, and that's what you need from an agent") and you go your separate ways, each to his or her next blind date.
The only difference is that the writer in this scenario is so desperate that he or she will start up a relationship with just about anybody...