  The van der Waals ferromagnet Fe$_3$GeTe$_2$ has recently attracted extensive
research attention due to its intertwined magnetic, electronic and topological
properties. Here, using high-resolution angle-resolved photoemission
spectroscopy, we systematically investigate the temperature evolution of the
electronic structure of bulk Fe$_3$GeTe$_2$. We observe largely dispersive
energy bands that are narrowed by a factor of 1.6 compared with ab-initio
calculation. Upon heating towards the ferromagnetic transition near 225 K, we
observe a massive reduction of quasiparticle coherence in a large energy range,
which is attributed to the enhanced magnetic fluctuation in the system.
Remarkably, the electron bands barely shift with increasing temperature, which
deviates from the exchange splitting picture within the itinerant Stoner model.
We argue that the local magnetic moments play a crucial role in the
ferromagnetism of Fe$_3$GeTe$_2$, despite its strongly itinerant nature. Our
results provide important insights into the electronic and magnetic properties
of Fe$_3$GeTe$_2$ and shed light on the generic understanding of itinerant
magnetism in correlated materials.
