What is the difference between a submissive slave and masochist man?

There are two differences: Being a submissive slave is a choice. Being a Masochist is not.

Another difference is that being a submissive slave is is nothing but roleplay that they enjoy doing. They have no limits and will do anything to satisfy their 'Master'. A Masochist, however, gains pleasure by being abused/humiliated/downgraded, but isn't after lust in particular.

Masochism usually develops at a very young age. It can have many reasons: It can be because - when/if your parents spanked you across their lap - you were told that recieving a punishment is the only way to make up for being disobedient.

Another reason can be because were being bullied/humiliated in school.

Then of course, another reason can be because you were raped/sexually assaulted as a child.

One way or the other, masochism is the result of being downgraded, controlled, humiliated or (mentally) abused in the past, people that suffer from this disorder (yes, it's actually considered as a mental disorder) have a very low self-esteem.

Hopefully this answers your question. :)