  In this letter we report a thorough analysis of the exciton dispersion in
bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation
at finite $\mathbf{q}\parallel \Gamma K$, and we compare our results with
recent high-accuracy electron energy loss data. Simulations reproduce the
measured dispersion and the variation of the peak intensity. We focus on the
evolution of the intensity, and we demonstrate that the excitonic peak is
formed by the superposition of two groups of transitions that we call $KM$ and
$MK'$ from the k-points involved in the transitions. These two groups
contribute to the peak intensity with opposite signs, each damping the
contributions of the other. The variations in number and amplitude of these
transitions determine the changes in intensity of the peak. Our results
contribute to the understanding of electronic excitations in this systems along
the $\Gamma K$ direction, which is the relevant direction for spectroscopic
measurements. They also unveil the non-trivial relation between valley physics
and excitonic dispersion in h--BN, opening the possibility to tune excitonic
effects by playing with the interference between transitions. Furthermore, this
study introduces analysis tools and a methodology that are completely general.
They suggest a way to regroup independent-particle transitions which could
permit a deeper understanding of excitonic properties in any system.
