In Metropolis - Hasting sampling, the proposal distribution does not necessarily have to have a form similar to that of the target distribution from which attempts are made to sample from. For one, I understand that a symmetrical choice of proposal kernel can vastly simplify the conditional proposal which yield a simple ratio of target distribution but this says more about the simplification of the acceptance ratio rather than the goodness of a proposal distribution to a target distribution.
What motivates our choice of proposal distribution for efficient sampling?