The flea lives on the body of its host and feeds on its blood. why is it in the best interest of the flea not to kill its host?

If the flea were to kill the host, it wouls have to do extra work to find another food source because it just eliminated the food source or host it had previously found. To clarify, it is not in its best interest to kill the host because it wants/needs to continue to use it as a food source instesd of having to find another one.