  The attractive interaction between $J/\psi$ and $\psi(3770)$ has to be strong
enough to form the molecular resonant structure, i.e., $X(6900)$. We argue that
since $\psi(3770)$ decays predominantly into a $D\bar D$ pair, the interactions
between $J/\psi$ and $\psi(3770)$ may be significantly enhanced due to the
three point $D\bar D$ loop diagram. The enhancement comes from the anomalous
threshold located at $t=-1.288$GeV$^2$, which effect propagates into the
$s$-channel partial wave amplitude in the vicinity of $\sqrt{s}\simeq 6.9$GeV.
This effect may be helpful in forming the $X(6900)$ peak.
