Our Lord seems to speak here of two different kinds of servants. Some will serve freely out of love; others servilely from fear. If one serves a master out of love, it necessarily follows, that he will hate the other. If one serves out of fear, it must follow that while he bears with one, he will not have any regard for the other.
Not only do we have a choice in the master that we will serve, we also have a choice in how we will serve him. It is clear from the Gospel that we should choose to serve God rather than mammon. (Mammon is a Syrian word for riches, but it also refers to the devil; who is the "prince of this world".) In choosing to serve God, we can serve as loving children or as servile slaves.
Many throughout the Old Testament obeyed God out of fear rather than love. Their obedience was servile and therefore they were often found obeying the letter of the Law rather than the Spirit. They likewise did not actually hate mammon but became indifferent to it. With the coming of Christ we are reminded that God is our Father, and we are His children. Jesus has become one with us, so that we can become one with God. In this change of relationships, we are shown that God does not desire slavish obedience in fear, but rather loving obedience of children. This relationship of love demands that we now hate mammon.
If we love money, the world, etc. then we will necessarily hate God. The Scriptures tell us that the love of money (covetousness) is the root of all evils. This is probably not the most popularly chosen path of men. It is serving the master mammon through fear that is perhaps most common. Greed and avarice are often motivated by a fear of not having enough. Men see the foolishness of loving mammon, but they often cannot overcome the temptation to serve mammon through fear. Our society instils a constant fear, so that we work to save enough to retire, we save for a rainy day, we buy insurance in case of an unforeseen expense, etc. It is this fear that enslaves so many to mammon.
Those who are serving mammon through fear, do not necessarily hate God, they rather just do not consider God at all. He is forgotten. They become indifferent to Him. This seems to be the state of affairs with most of the people in our society today. God is not hated, but neither is He loved. These people neither fight for or against God. They are perhaps the greatest atheists. (The atheist that fights against God, of necessity proves that he believes in Him. He can only fight against things that exist. If God does not exist then he could not fight against Him.) God can work with those that either love Him or hate Him (they are either hot or cold); but God can do nothing with the lukewarm or the indifferent.