  The effective interactions between two nuclear clusters, $d+d$, $t+t$, and
$\alpha+\alpha$, are investigated within a cluster model using local
nucleon-nucleon~($NN$) forces. It is shown that the interaction in the
spin-aligned $d+d$ system is repulsive for all inter-cluster distances, whereas
the $\alpha+\alpha$ and spin-aligned $t+t$ systems are attractive at
intermediate distances. The Pauli blocking between identical-nucleon pairs is
responsible for the cluster-cluster repulsion and becomes dominant in the
shallow binding limit. We demonstrate that two $d$-clusters could be bound if
the $NN$ force has nonzero range and is strong enough to form a deeply bound
$d$-cluster, or if the $NN$ force has both even-parity and odd-parity
attraction. Effective dimer-dimer interactions for general quantum systems of
two-component fermions are also discussed in heavy-light mass limit, where one
component is much heavier than the other, and their relation to inter-cluster
interactions in nuclear systems are discussed. Our findings provide a
conceptual foundation for conclusions obtained numerically in the literature,
that increasing the range or strength of the local part of the attractive
nucleon-nucleon interaction results in a more attractive cluster-cluster
interaction.
