A post from US agent Rachelle Gardner about why agents don't give personalised feedback with each rejection letter:
Once you've read it, what I'm about to say can be taken in context ... Agents (and publishers/editors) make decisions about submissions very quickly. Probably more quickly than you would like. Just as Rachelle suggested, it's analogous to deciding whether or not you like a piece of clothing in a shop. I've seen enough queries in my life to know very quickly if something's going to be right for me. I'm not going to read every last word before coming to this conclusion, so I couldn't give a personalised rejection even if I want to. I give them to some people, but not everyone. In truth, I'd love to be able to give constructive feedback but I simply can't. The day is but twenty-four hours long. And there's another reason why we can't spend the time to give personalised rejections: our existing clients are subsidising the time we spend reviewing submissions. So they have to come first.
Thus, we've established that agents simply can't give detailed feedback on everything we read, yet you'd be surprised how many submissions I receive which are actually demands - not requests - for feedback. (Unsurprisingly, these receive a 'no thanks' fairly quickly.) Sometimes I suspect that the individuals involved simply don't understand what agents do, but there's so much information available these days that there's no excuse to not know. There are, as Rachelle says, editors and manuscripts assessors and writers groups and writers centres out there to help you assess your manuscript. The agent is who you come to after that point. Unless you'd like to think up a way for us to get a nifty amount of funding in order to employ ten more people each to read all the submissions and give feedback on every single one of them.