From my perspective, your premise or the way you're framing the issue is wrong. I think the Democrats are wrong about some things - a lot of things, actually. But I think the Republicans are seriously wrong about everything. See, I don't always vote for Democrats as much as I vote against Republicans.
The Republicans aren't just wrong in terms of policy; I see them as a party that fundamentally wants to impose a radical form of social, economic, and political austerity on the rest of us against our will. I know you don't agree, which is fine. I don't expect people to agree with me. But if you want to understand my position, that's it.
If the Republican party weren't such an existential threat to the future of this country, I would have no problem voting for an occasional R or an Independent or even sit out an election. But the stakes are too high for that. I can't not vote Democratic because the Republican party must be defeated and the Democratic candidates are usually the ones who can deliver that result.