So read, and learn what the Civil War (or the War Between the States) was all about...
But probably the best overall source of original materials - letters, journals, etc. - that details why these men in Blue and Grey fought and died, is contained in James McPherson's 1997 book - Causes & Comrades: Why Men Fought In The Civil War (ISBN: 0195090233). Some of the letters contained in this book will make your heart swell and your eyes water, others will make you wonder how Americans could believe such things.
Origins of the Civil War
on the Wikipedia site
Don't Know Much About the Civil War:
Everything You Need to Know About America's Greatest Conflict but Never Learned
by Kenneth C. Davis.
Or you might want to read what Alexander Stephens, the Vice-President of the Confederate States of America, told a southern audience on March 21, 1861.
His complete speech is available at: http://teachingamericanhistory.org/library/index.asp?document=76.
His complete speech and introductory remarks as reported in the Savannah Republican, is also available at http://civilwarcauses.org/corner.htm.
Battle of Olustee home page