Most things we buy have a guarantee. So it's natural to expect one when you hire a lawyer. Not only do people want a guarantee - they want to know how their case is going to turn out. You can't blame people. The biggest creator of stress is not knowing what is going to happen. That's true for most things in life. It's even more true when the possibilities include the loss of your freedom. No matter how bad you want one - and how badly the lawyer wants to provide one, no lawyer can provide a guarantee.
There are a number of reasons for this. The biggest one is that there are simply too many unknowns. When the lawyer first meets a client they usually don't know anything about the case, which means they have to rely on the client. No one can be entirely objective when you have a personal interest in the matter; you tend to view things in the light most favorable to you. Additionally, in some cases the client may not know everything about the case; that's usually the case when they are arrested following some type of investigation.
In addition to not knowing all the facts, you also don't know who is going to prosecute the case, or what judge is going to handle it. Different prosecutors look at cases at differently; some deal harshly with some types of crimes, while others may be more lenient.
In addition to all of the above considerations, things can change while the case is progressing toward trial. The State may continue their investigation, and the defense lawyer is certainly going to investigate; at least any good lawyer will do so.
Some cases end up going to trial. If a lawyer has any experience at all, they know that no one can successfully predict what a jury will do. What lawyers who have any experience have won cases they thought they would lose, and lost cases they were sure they would win. If the case was so clear that you knew what a jury would do you probably wouldn't be trying it in the first place.
Even though no lawyer can guarantee what will happen, that doesn't prevent some lawyers from doing it. Times are tough, and there's a lot of competition among lawyers. Some lawyers don't mind making guarantees to try and get business. They'll worry not being able to fulfill it later. This should be a giant red flag - if you talk to a lawyer who makes a guarantee, run as fast as you can.
Unfortunately, I've spent a lot of time at different doctor's offices - including quite a few specialists. My wife fought cancer for several years before passing away in 2010. Going to different doctors was a regular occurence, especially toward the end. You go to them because you want to make sure you receive the best possible care; you don't ask for a guarantee, and don't expect one. Why? Because you know they can't predict what is going to happen. No matter how smart they are, and how much experience they have, they can't successfully predict how things are going to turn out.
Lawyers are no different - there are too many variables to be able to successfully predict what is going to happen. We may be have a pretty good idea, but it is nothing more than a guess. All we can guarantee is that we will do the best job we can, using all the experience and knowledge we possess.
So when you are choosing a lawyer, select the one you believe is doing the best job they can. That's all any lawyer can guarantee.