  The classical Hamiltonian for a chargeless and massless particle in a very
near horizon region is shown to be of the form $H\sim xp$ as long as radial
motion is concerned. This is demonstrated explicitly for static spherically
symmetric black hole and also found to be applicable for specific choice of
radial trajectories in the Kerr case. Such feature of horizon leads to
unavoidable "{\it local instability}" in the particle's radial motion as long
as near horizon regime is concerned. We show that at the quantum level this
provides thermality in the system. The temperature is found to be given by the
Hawking expression. Finally, we conjecture that the automatic instability
created by the horizon is responsible for its own temperature and consequently
can be a possible physical mechanism for horizon temperature.
