Men fight for freedom because without freedom their loved ones have no quality of life. Also, the men hope they will not die and will be able to enjoy the fruits of their fight if they survive. Anything cornered will fight. All creatures need to be free.
The brief answer to this is because freedom is important. One of the main things that people want is to be in control of their own lives. That is (you can argue) the basis even for such things as religion -- we try to get God or the gods to do what we want. The point is, we really want to be in control and freedom is a way of being in control of your own life rather than doing what others tell you to do.