I'd probably start with the question of: isn't it easier to understand the causal relationships in a lot of diseases than it is to understand behavior, the biology of which we barely understand without bringing genetics into it?
However, I think our techniques are quite sufficient right now to say X% of the variance in phenotype P is attributable to genetics (i.e., genetics partially cause P). That is a totally different thing from explaining how the genotype causes the phenotype.