why is that most of the books in mathematics don't include answers? I am currently going through the " Topics in Algebra" By I. N. Herstein. The problems are pretty good. But there are no answers on the back. Same is the case with "Mathematical Analysis" by Rudin.
I don't understand why is this so??
Even though some would argue that it robs the question of its charm, sometimes it is necessary to check out the answer just to get confirmation that what you have done is correct.
Moreover it becomes necessary in an environment where you study on your own. Even though I have solved exercises, there is always a doubt regarding the correctness of the process or answer.
 A: I think it is a commercial decision.  Professors prefer it this way. 
I personally think having the answers is beneficial.  Even if you are confident in your methods, you may have made a dumb mistake (or have a flawed understanding).  Usually if you are on top of the course, you can check the answer and if wrong, rework the problem correctly (if needed, checking the book).  Also, working problems can be a chore--having immediate feedback is good psychology.
Note that many classic books have the answers in the back (Poly anything, Granville calculus, Hart College Algebra).  This used to be much more the norm.  I think the change is more of a commercial one and one that emphasizes the primacy of gatekeepers rather than of learning.  [I'm sure the professors tell themselves they care about learning and are warding off lazy students, but how many of them really collect and grade daily homework in sufficient quantities to drive needed learning?]
A: Several answers to this question come to my mind, some of which are already covered by the comments. Actually, if I'd ever were to write a textbook I'd do the same. 
Since you cite Rudin I'd like to note in advance that many consider his books on analysis among the best, and I don't think it is a coincidence that someone writing books which do get that attention and appreciation follows this practice.
Something everyone learning mathematics (not only there) has to learn is the reality (from which you are well protected at school) that you will usually (not sometimes, usually) face problems which you cannot solve. Maybe not at all, at least not with the first attempts or with little effort. You have to find your way to the solution one way or the other, either by thinking harder and smarter, by talking to people, whatever. But you don't get it for free, in real life. Almost never. It is an experience of people who teach (not just math), that this fact has to be taught as well, unless they want to produce many many people leaving university who will then have to learn it too late the hard way. 
And, no insult intended, one word of advice: if you find you really need the solutions to make your way through such books, you may want to consider whether you've really chosen the right topic for you.
And one additional remark: why do you need the solution? By working seriously on a problem you may learn more -- even if you cannot solve it -- than by looking up the answer (and, very likely, missing most of the subtleties).
A: I think Maths textbooks should, at the very least least, provide hints on how to tackle the excercises. I disagree with the argument that students of maths will benefit from the "life lesson" that they aren't as smart as they'd like to be. Most of them have enough humility to know that already. Very often, the exercises make mathematical points that do not come across from the main text, and if the student cannot tackle the exercise, he/she will miss out.
The fact is that most students of maths are not destined to be professors of the subject. Nevertheless they need to gain a good understanding of the subject in a limited space of time and do not always have access to a tutor. When they encounter a textbook exercise they cannot complete, they will lose confidence in a book they have paid a lot of money for and, even worse, they lose confidence in themselves.
Any maths textbook that causes a student to lose confidence, is a failure, no matter how eminent the author.
Many years ago, when I was a first-year undergraduate in maths, one of my lecturers kicked off his course by saying "The first rule of buying mathematics books is don't bother". He then went on to say that most of them are written by second-raters who need the money. Cynical perhaps, but I do think there is an arrogance amongst some authors who have little or no empathy for the average student for whom maths is just one of many subjects that he or she has to master.
