From this agent's point-of-view:
I have had clients turn down book offers. I don't always agree with their reasons but my ultimate goal includes working for that author, so it behooves me to listen. In one case, it then took me two and a half years to sell that same book elsewhere (with no other sales in between). That was a long and frustrating journey -- for the author, too, I'm sure.
Reasons we have turned down offers:
Thought we could get a better deal elsewhere -- Thank you, Captain Obvious. However.... do listen to your agent in this instance as they are far more likely to know whether a better offer is reasonable to expect. What you want (what your agent wants) and what is reasonable for the market to bear are not always the same.
Overcommitment. Not that I'm going to point any fingers or name any names, but I have occasionally had clients who want to accept an offer even when there is no human way that they will ever deliver on time. An author exhausting themselves does not lead to their best work. One of the things an agent can do is help to manage your career and that includes knowing your ability to produce and what effects it may have on the other parts of your life.
Author has had problems with the publisher in the past -- Again, listen to your agent here. If the staff has had some change-over, your issues may no longer be relevant in the new regime. Also, a caution -- if an agent begins to get the impression that it's the author's approach that needs mending, they do have their entire client list to consider and their own relationship with the house as well. This is unavoidable. So, think carefully before you act in the publishing world.
I would never, ever, ever turn down an offer without first consulting the client (though I have heard from some writers that there are agents out there who have done that). I might very well tell them up front why I don't think the offer is good for them, but the final decision must be the author's.
The first offer of representation may not always be your best one either. Yes, I said that out loud. There are a lot of things to consider when choosing an agent -- not just whether they are the first to say yes. I advocate having a conversation with them (either by phone or email if you haven't already met them) and trying to get a feel for them as a person. With any luck you will be working together for a very long time over the course of many books. Think of them as a nanny who will raise your children. And then think of all those Lifetime movies that are about evil nannies that steal your children or kill them. Hmmm.... What a strange analogy I've stumbled across. In any case, I have an entire workshop based on the idea of choosing an agent who is right for you and considering different aspects of their workstyle, personality, and so forth. How do you find this out? By being connected in the writer community mostly. But, be cautious.... There are some sour grapes out there who will falsely malign an agent's reputation, so don't just take one person's word for anything. Do your homework.
It would be so ideal if every talented writer could find the exact right agent and exact right publisher the first time around.....