This is a difficult subject to discuss in quick posts, so I encourage you to go and read a book such as The Road to Serfdom, as a start.
However, think of what is actually going on:
-A bunch of people collectively decide that how much money other people should provide them, so that they can facilitate those funds.
-This bunch of people sell this plan on the idea that they will do what is for the good of the many.
-Person A is forced to give up their money, to this group. If Person A refuses to do so, this group will imprison Person A.
The freedom that a person otherwise would have, to decide how one wishes to distribute his/her funds, is being taken away, by force, by a group. This is a clear reduction in freedom.