As far as being rich goes, I think that house elves must be very expensive to buy, unless it is being handed down through inheritance. I think that once, they were free and lived secluded lives from wizards but then when wizards found them, the wizards assumed that they were better than the elves. They then put several spells on the elves (possibly the Imperius curse) to force them to to do the wizard's bidding. After thousands of years of this, the elves have been brainwashed to believe that this is what they were meant to do. The reason they enjoy work is because they feel that that is what they were born to do. They feel that they have no reason to live if they don't have housework to do.