Why did thomas hobbes believed that people needed an absolute ruler?

  Because human beings in their natural condition are trapped in competition and struggle with each other, they need a strong, absolute ruler in order to provide security and stability to society.

Details:

Thomas Hobbes published a famous work called Leviathan in 1651.  The title "Leviathan" comes from a biblical word for a great and mighty beast.  Hobbes believed government is formed by people for the sake of their personal security and stability in society.  In Hobbes' view, once the people put a king (or other leader in power), then that leader needs to have supreme power (like a great and mighty beast).    Hobbes' view of the natural state of human beings without a government held that people are too divided and too volatile as individuals -- everyone looking out for his own interests.  So for security and stability, authority and the power of the law needs to be in the hands of a powerful ruler like a king or queen.  And so people willingly enter a "social contract" in which they live under a government that provides stability and security for society.

Probably the most famous set of lines from Hobbes' Leviathan book describes what he saw as the natural state of human affairs without government -- one in which every individual had freedom, but that meant it was a situation of "war of all against all," or we might say, every man for himself.  Hobbes wrote:

there were abundant sources of fuel nearby