Every writer I know has enough rejection letters from agents to wallpaper their entire house. This is a rite of passage. Another finally published author I know went through 400 agents before he received a contract. And look at Jack Canfield (Chicken Soup for the Soul) - his book was rejected by 250 publishers before one said, "Let's give it a shot."
Some of the rejections are form letters copied hundreds of times and so bad they are hard to read.
In any event, the personalized rejection letters (not the form letters) can take the temperature of your work. Most of the time you have to read between the lines to get the information, but it usually is a good gauge.
For example, if an agent is really excited about your query letter and asks for a partial, then rejects your work, it could be your writing that didn't appeal to them. Likewise, if a full manuscript is read and rejected perhaps the mechanics of the novel didn’t work for them.
If the agent says "not right for them" or "I don't think I could sell this" it may mean the market is already flooded with similar stories. Agents talk to publishing house editors all the time and the editors tell them what is selling and what is not. I know of one agent who said that sometimes she has to reject wonderful work because it would not sell.
Still don't take this advice as the absolute truth - it is still only the agent's opinion and another agent may think your work is fantastic.
Getting published is a matter of:
- Good writing
- A good storyline that has not been done before or one that has a different twist
- Timing - the market is reading your genre - (women’s literature seems to be all the rage right now along with mysteries and thrillers with a female protagonist)
- Luck in finding an agent who loves your work
In any event, keep trying.