Why is the flow of signals in a synapse from axonal end of one neuron to dendritic end of another neuron but not the reverse?

Axonal terminals have neurotransmitters which are chemical substances. They diffuse across the synapse and reach the dendrite of next neuron. This induces the impulse in the dendrites of next neuron. These neurotransmitters are absent in dendrites of the neuron. Synapse act as one way valve so the flow of signals in a synapse occurs from axonal end of one neuron to dendrite end of another neuron but not the reverse.