You should strive to be the thinker of the thought. Doing so makes you the master of your mind, controlling what you think about. Much more common however is when people become servants to their mind.
They let thoughts, memes, and beliefs drift in or be placed there by others. They become prisoners of a negative mindset, and live lives of diminished expectation and unfulfilled potential.
Your mind makes a cruel master, but a generous servant.